ATOMIC ENERGY
FOR MILITARY PURPOSES
Copyright, 1945, by H. D. Smyth
Reproduction in whole or in part
authorized and permitted
Printed in the United States of America
by Maple Press, York, Pennsylvania
FOREWORD
THE story of the development of the atomic bomb by the
combined efforts of many groups in the United States is a
fascinating but highly technical account of an enormous enter-
prise. Obviously military security prevents this story from being
told in full at this time. However, there is no reason why the
administrative history of the Atomic Bomb Project and the basic
scientific knowledge on which the several developments were
based should not be available now to the general public. To this
end this account by Professor H. D. Smyth is presented.
All pertinent scientific information which can be released to
the public at this time without violating the needs of national
security is contained in this volume. No requests for additional
information should be made to private persons or organizations
associated directly or indirectly with the project. Persons dis-
closing or securing additional information by any means whatso-
ever without authorization are subject to severe penalties under
the Espionage Act.
The success of the development is due to the many thousands
of scientists, engineers, workmen and administrators both
civilian and military whose prolonged labor, silent perseverance,
and whole-hearted cooperation have made possible the un-
precedented technical accomplishments here described.
L. R. GROVES
Major General, USA
War Department
Washington, D. C.
August 1945
PREFACE
THE ultimate responsibility for our nation's policy rests on
its citizens and they can discharge such responsibilities wisely
only if they are informed. The average citizen cannot be expected
to understand clearly how an atomic bomb is constructed or how
it works but there is in this country a substantial group of engi-
neers and scientific men who can understand such things and
who can explain the potentialities of atomic bombs to their fellow
citizens. The present report is written for this professional group
and is a matter-of-fact, general account of work in the United
States since 1939 aimed at the production of such bombs. It is
neither a documented official history nor a technical treatise for
experts. Secrecy requirements have affected both the detailed
content and general emphasis so that many interesting develop-
ments have been omitted.
References to British and Canadian work are not intended to
be complete since this is written from the point of view of the
activities in this country.
The writer hopes that this account is substantially accurate,
thanks to cooperation from all groups in the project; he takes
full responsibility for such errors as may occur.
H. D. SMYTH
July /, 1945
Minor changes have been made for this edition. These changes
consist of the following variations from the report as issued
August 12, 1945: (1) Minor clarifications and corrections in
wording; (2) Inclusion of a paragraph on radioactive effects
issued by the War Department to accompany the original release
viii Preface
of this report; (3) Addition of a few sentences on the success of
the health precautions; (4) Addition of a few names; (5) Addition
of Appendix 6, giving the War Department release on the New
Mexico test of July 16, 1945; (6) Inclusion of the photographic
section; (7) Inclusion of an index.
H. D. S.
September 7, 1945
CONTENTS
FOREWORD v
PREFACE vii
I. INTRODUCTION 1
II. STATEMENT OF THE PROBLEM 31
III. ADMINISTRATIVE HISTORY UP TO
DECEMBER 1941 45
IV. PROGRESS UP TO DECEMBER 1941 55
V. ADMINISTRATIVE HISTORY 1942-1945 75
VI. THE METALLURGICAL PROJECT AT
CHICAGO IN 1942 88
VII. THE PLUTONIUM PRODUCTION PROBLEM
AS OF FEBRUARY 1943 108
VIII. THE PLUTONIUM PROBLEM, JANUARY 1943
TO JUNE 1945 130
IX. GENERAL DISCUSSION OF THE SEPARATION
OF ISOTOPES 154
X. THE SEPARATION OF THE URANIUM ISOTOPES
BY GASEOUS DIFFUSION 172
XL ELECTROMAGNETIC SEPARATION OF URANIUM
ISOTOPES 187
XII. THE WORK ON THE ATOMIC BOMB 206
XIII. GENERAL SUMMARY 223
APPENDICES 227
INDEXES 255
ILLUSTRATION SECTION IS IN CHAPTER VIII
ATOMIC ENERGT
FOR MILITARY PURPOSES
CHAPTER I. INTRODUCTION
1.1. The purpose of this report is to describe the scientific and
technical developments in this country since 1940 directed toward
the military use of energy from atomic nuclei. Although not
written as a "popular" account of the subject, this report is
intended to be intelligible to scientists and engineers generally
and to other college graduates with a good grounding in physics
and chemistry. The equivalence of mass and energy is chosen as
the guiding principle in the presentation of the background
material of the "Introduction."
THE CONSERVATION
OF MASS AND OF ENERGY
1.2. There are two principles that have been cornerstones of
the structure of modern science. The first that matter can be
neither created nor destroyed but only altered in form was
enunciated in the eighteenth century and is familiar to every
student of chemistry; it has led to the principle known as the law
of conservation of mass. The second that energy can be neither
created nor destroyed but only altered in form emerged in the
nineteenth century and has ever since been the plague of in-
ventors of perpetual-motion machines; it is known as the law of
conservation of energy.
1.3. These two principles have constantly guided and disci-
plined the development and application of science. For all
practical purposes they were unaltered and separate until some
five years ago. For most practical purposes they still are so, but
it is now known that they are, in fact, two phases of a single
principle for we have discovered that energy may sometimes be
converted into matter and matter into energy. Specifically, such
a conversion is observed in the phenomenon of nuclear fission of
1
2 Introduction
uranium, a process in which atomic nuclei split into fragments
with the release of an enormous amount of energy. The military
use of this energy has been the object of the research and pro-
duction projects described in this report.
THE EQUIVALENCE
OF MASS AND ENERGY
1.4. One conclusion that appeared rather early in the develop-
ment of the theory of relativity was that the inertial mass of a
moving body increased as its speed increased. This implied an
equivalence between an increase in energy of motion of a body,
that is, its kinetic energy, and an increase in its mass. To most
practical physicists and engineers this appeared a mathematical
fiction of no practical importance. Even Einstein could hardly
have foreseen the present applications, but as early as 1905 he
did clearly state that mass and energy were equivalent and sug-
gested that proof of this equivalence might be found by the study
of radioactive substances. He concluded that the amount of
energy, E, equivalent to a mass, m, was given by the equation
E = mc 2
where c is the velocity of light. If this is stated in actual numbers,
its startling character is apparent. It shows that one kilogram
(2.2 pounds) of matter, if converted entirely into energy, would
give 25 billion kilowatt hours of energy. This is equal to the
energy that would be generated by the total electric power
industry in the United States (as of 1939) running for approxi-
mately two months. Compare this fantastic figure with the 8.5
kilowatt hours of heat energy which may be produced by burning
an equal amount of coal.
1.5. The extreme size of this conversion figure was interesting
in several respects. In the first place, it explained why the equiva-
lence of mass and energy was never observed in ordinary chemical
combustion. We now believe that the heat given off in such a
combustion has mass associated with it, but this mass is so small
that it cannot be detected by the most sensitive balances avail-
Introduction 3
able. (It is of the order of a few billionths of a gram per mole.)
In the second place, it was made clear that no appreciable
quantities of matter were being converted into energy in any
familiar terrestrial processes, since no such large sources of energy
were known. Further, the possibility of initiating or controlling
such a conversion in any practical way seemed very remote.
Finally, the very size of the conversion factor opened a magnifi-
cent field of speculation to philosophers, physicists, engineers,
and comic-strip artists. For twenty-five years such speculation
was unsupported by direct experimental evidence, but beginning
about 1 930 such evidence began to appear in rapidly increasing
quantity. Before discussing such evidence and the practical partial
conversion of matter into energy that is our main theme, we shall
review the foundations of atomic and nuclear physics. General
familiarity with the atomic nature of matter and with the exist-
ence of electrons is assumed. Our treatment will be little more
than an outline which may be elaborated by reference to books
such as Pollard and Davidson's Applied Nuclear Physics and Strana-
than's The "Particles" of Modern Physics.
RADIOACTIVITY AND
ATOMIC STRUCTURE
1.6. First discovered by H. Becquerel in 1896 and subsequently
studied by Pierre and Marie Curie, E. Rutherford, and many
others, the phe'nomena of radioactivity have played leading roles
in the discovery of the general laws of atomic structure and in the
verification of the equivalence of mass and energy.
lONIZATION BY RADIOACTIVE SUBSTANCES
1.7. The first phenomenon of radioactivity observed was the
blackening of photographic plates by uranium minerals. Al-
though this effect is still used to some extent in research on radio-
activity, the property of radioactive substances that is of greatest
scientific value is their ability to ionize gases. Under normal con-
ditions air and other gases do not conduct electricity otherwise
power lines and electrical machines would not operate in the
4 Introduction
open as they do. But under some circumstances the molecules
of air are broken apart into positively and negatively charged
fragments, called ions. Air thus ionized does conduct electricity.
Within a few months after the first discovery of radioactivity
Becquerel found that uranium had the power to ionize air.
Specifically he found that the charge on an electroscope would
leak away rapidly through the air if some uranium salts were
placed near it. (The same thing would happen to a storage
battery if sufficient radioactive material were placed near by.)
Ever since that time the rate of discharge of an electroscope has
served as a measure of intensity of radioactivity. Furthermore,
nearly all present-day instruments for studying radioactive phe-
nomena depend on this ionization effect directly or indirectly.
An elementary account of such instruments, notably electro-
scopes, Geiger-Muller counters, ionization chambers, and Wilson
cloud chambers is given in Appendix 1.
THE DIFFERENT RADIATIONS OR PARTICLES
1.8. Evidence that different radioactive substances differ in
their ionizing power both in kind and in intensity indicates that
there are differences in the "radiations" emitted. Some of the
radiations are much more penetrating than others; consequently,
two radioactive samples having the same effect on an "un-
shielded" electroscope may have very different effects if the
electroscope is "shielded," i.e., if screens are interposed between
the sample and the electroscope. These screens are said to absorb
the radiation.
1.9. Studies of absorption and other phenomena have shown
that in fact there are three types of "radiation" given off by
radioactive substances. There are alpha particles, which are
high-speed ionized helium atoms (actually the nuclei of helium
atoms), beta particles, which are high-speed electrons, and
gamma rays, which are electromagnetic radiations similar to
X-rays. Of these only the gamma rays are properly called radia-
tions, and even these act very much like particles because of
their short wave-length. Such a "particle" or quantum of gamma
Introduction 5
radiation is called a photon. In general, the gamma rays are very
penetrating, the alpha and beta rays less so. Even though the
alpha and beta rays are not very penetrating, they have enormous
kinetic energies for particles of atomic size, energies thousands
of times greater than the kinetic energies which the molecules of
a gas have by reason of their thermal motion, and thousands
of times greater than the energy changes per atom in chemical
reactions. It was for this reason that Einstein suggested that
studies of radioactivity might show the equivalence of mass and
energy.
THE ATOM
1.10. Before considering what types of atoms emit alpha, beta
and gamma rays, and before discussing the laws that govern such
emission, we shall describe the current ideas on how atoms are
constructed, ideas based partly on the study of radioactivity.
1.11. According to our present view every atom consists of a
small heavy nucleus approximately 10~~ 12 cm in diameter sur-
rounded by a largely empty region 10~ 8 cm in diameter in which
electrons move somewhat like planets about the sun. The nucleus
carries an integral number of positive charges, each 1.6 X 10~ 19
coulombs in size. (See Appendix 2 for a discussion of units.)
Each electron carries one negative charge of this same size, and
the number of electrons circulating around the nucleus is equal
to the number of positive charges on the nucleus so that the atom
as a whole has a net charge of zero.
1.12. Atomic Number and Electronic Structure. The number of
positive charges in the nucleus is called the atomic number, Z.
It determines the number of electrons in the extranuclear struc-
ture, and this in turn determines the chemical properties of the
atom. Thus all the atoms of a given chemical element have the
same atomic number, and conversely all atoms having the same
atomic number are atoms of the same element regardless of
possible differences in their nuclear structure. The extranuclear
electrons in an atom arrange themselves in successive shells
according to well-established laws. Optical spectra arise from
6 Introduction
disturbances in the outer parts of this electron structure; X-rays
arise from disturbances of the electrons close to the nucleus. The
chemical properties of an atom depend on the outermost elec-
trons, and the formation of chemical compounds is accompanied
by minor rearrangements of these electronic structures. Conse-
quently, when energy is obtained by oxidation, combustion,
explosion, or other chemical processes, it is obtained at the
expense of these structures so that the arrangement of the elec-
trons in the products of the process must be one of lowered energy
content. (Presumably the total mass of these products is corre-
spondingly lower but not detectably so.) The atomic nuclei are
not affected by any chemical process.
1.13. Mass Number. Not only is the positive charge on a nucleus
always an integral number of electronic charges, but the mass
of the nucleus is always approximately a whole number times a
fundamental unit of mass which is almost the mass of a proton,
the nucleus of a hydrogen atom. (See Appendix 2.) This whole
number is called the mass number, A, and is always at least twice
as great as the atomic number except in the cases of hydrogen
and a rare isotope of helium. Since the mass of a proton is about
1,800 times that of an electron, the mass of the nucleus is very
nearly the whole mass of the atom.
1.14. Isotopes and Isobars. Two species of atoms having the
same atomic number but different mass numbers are called
isotopes. They are chemically identical, being merely two species
of the same chemical element. If two species of atoms have the
same mass number but different atomic numbers, they are called
isobars and represent two different chemical elements.
RADIOACTIVITY AND NUCLEAR CHANGE
1.15. If an atom emits an alpha particle (which has an atomic
number of two and a mass of four), it becomes an atom of a
different element with an atomic number lower by two and a
mass number lower by four. The emission by a nucleus of a beta
particle increases the atomic number by one and leaves the mass
number unaltered. In some cases, these changes are accompanied
Introduction 7
by the emission of gamma rays. Elements which spontaneously
change or "disintegrate" in these ways are unstable and are
described as being "radioactive." The only natural elements
which exhibit this property of emitting alpha or beta particles
are (with a few minor exceptions) those of very high atomic
numbers and mass numbers, such as uranium, thorium, radium,
and actinium, i.e., those known to have the most complicated
nuclear structures.
HALF-LIVES; THE RADIOACTIVE SERIES
1.16. All the atoms of a particular radioactive species have the
same probability of disintegrating in a given time, so that an
appreciable sample of radioactive material, containing many
millions of atoms, always changes or "disintegrates" at the same
rate. This rate at which the material changes is expressed in
terms of the "half-life," the time required for one half the atoms
initially present to disintegrate, which evidently is constant for
any particular atomic species. Half-lives of radioactive materials
range from fractions of a second for the most unstable to billions
of years for those which are only slightly unstable. Often, the
"daughter" nucleus like its radioactive "parent" is itself radio-
active and so on down the line for several successive generations
of nuclei until a stable one is finally reached. There are three
such families or series comprising all together about forty different
radioactive species. The radium series starts from one isotope of
uranium, the actinium series from another isotope of uranium,
and the thorium series from thorium. The final product of each
series, after ten or twelve successive emissions of alpha and beta
particles, is a stable isotope of lead.
FIRST DEMONSTRATION OF ARTIFICIAL
NUCLEAR DISINTEGRATION
1.17. Before 1919 no one had succeeded in disturbing the
stability of ordinary nuclei or affecting the disintegration rates
of those that were naturally radioactive. In 1919 Rutherford
showed that high-energy alpha particles could cause an alteration
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Introduction 9
in the nucleus of an ordinary element. Specifically he succeeded
in changing a few atoms of nitrogen into atoms of oxygen by
bombarding them with alpha particles. The process involved
may be written as
He 4 + N 14 -i O 17 + H 1
meaning that a helium nucleus of mass number 4 (an alpha
particle) striking a nitrogen nucleus of mass number 14 produces
an oxygen nucleus of mass number 17 and a hydrogen nucleus
of mass number 1. The hydrogen nucleus, known as the "proton,"
is of special importance since it has the smallest mass of any
nucleus. Although protons do not appear in natural radioactive
processes, there is much direct evidence that they can be knocked
out of nuclei.
THE NEUTRON
1.18. In the decade following Rutherford's work many similar
experiments were performed with similar results. One series of
experiments of this type led to the discovery of the neutron,
which will be discussed in some detail since the neutron is prac-
tically the theme song of this whole project.
1.19. In 1930 W. Bothe and H. Becker in Germany found that
if the very energetic natural alpha particles from polonium fell
on certain of the light elements, specifically beryllium, boron or
lithium, an unusually penetrating radiation was produced. At
first this radiation was thought to be gamma radiation although
it was more penetrating than any gamma rays known, and the
details of experimental results were very difficult to interpret on
this basis. The next important contribution was reported in 1932
by Irene Curie and F. Joliot in Paris. They showed that if this
unknown radiation fell on paraffin or any other hydrogen-
containing compound it ejected protons of very high energy.
This was not in itself inconsistent with the assumed gamma-ray
nature of the new radiation, but detailed quantitative analysis of
the data became increasingly difficult to reconcile with such an
hypothesis. Finally (later in 1932) J. Chadwick in England per-
10 Introduction
formed a series of experiments showing that the gamma-ray
hypothesis was untenable. He suggested that in fact the new
radiation consisted of uncharged particles of approximately the
mass of the proton, and he performed a series of experiments
verifying his suggestion. Such uncharged particles are now called
neutrons.
1.20. The one characteristic of neutrons which differentiates
them from other subatomic particles is the fact that they are
uncharged. This property of neutrons delayed their discovery,
makes them very penetrating, makes it impossible to observe
them directly, and makes them very important as agents in
nuclear change. To be sure, an atom in its normal state is also
uncharged, but it is ten thousand times larger than a neutron and
consists of a complex system of negatively charged electrons
widely spaced around a positively charged nucleus. Charged
particles (such as protons, electrons, or alpha particles) and
electromagnetic radiations (such as gamma rays) lose energy in
passing through matter. They exert electric forces which ionize
atoms of the material through which they pass. (It is such ioniza-
tion processes that make the air electrically conducting in the
path of electric sparks and lightning flashes.) The energy taken
up in ionization equals the energy lost by the charged particle,
which slows down, or by the gamma ray, which is absorbed. The
neutron, however, is unaffected by such forces; it is affected only
by a very short-range force, i.e., a force that comes into play
when the neutron comes very close indeed to an atomic nucleus.
This is the kind of force that holds a nucleus together in spite of
the mutual repulsion of the positive charges in it. Consequently
a free neutron goes on its way unchecked until it makes a
"head-on" collision with an atomic nucleus. Since nuclei are
very small, such collisions occur but rarely and the neutron
travels a long way before colliding. In the case of a collision of
the "elastic" type, the ordinary laws of momentum apply as
they do in the elastic collision of billiard balls. If the nucleus that
is struck is heavy, it acquires relatively little speed, but if it is a
proton, which is approximately equal in mass to the neutron,
Introduction 1 1
it is projected forward with a large fraction of the original speed
of the neutron, which is itself correspondingly slowed. Secondary
projectiles resulting from these collisions may be detected, for
they are charged and produce ionization. The uncharged nature
of the neutron makes it not only difficult to detect but difficult to
control. Charged particles can be accelerated, decelerated, or
deflected by electric or magnetic fields which have no effect on
neutrons. Furthermore, free neutrons can be obtained only from
nuclear disintegrations; there is no natural supply. The only
means we have of controlling free neutrons is to put nuclei in
their way so that they will be slowed and deflected or absorbed
by collisions. As we shall see, these effects are of the greatest
practical importance.
THE POSITRON AND THE DEUTERON
1.21. The year 1932 brought the discovery not only of the
neutron but also of the positron. The positron was first observed
by C. D. Anderson at the California Institute of Technology. It
has the same mass and the same magnitude of charge as the
electron, but the charge is positive instead of negative. Except
as a particle emitted by artificially radioactive nuclei, it is of
little interest to us.
1.22. One other major discovery marked the year 1932. H. C.
Urey, F. G. Brickwedde, and G. M. Murphy found that hydrogen
had an isotope of mass number 2, present in natural hydrogen
to one part in 5,000. Because of its special importance this heavy
species of hydrogen is given a name of its own, deuterium, and
the corresponding nucleus is called the deuteron. Like the alpha
particle the deuteron is not one of the fundamental particles but
does play an important role in certain processes for producing
nuclear disintegration.
NUCLEAR STRUCTURE
1.23. The idea that all elements are made out of a few funda-
mental particles is an old one. It is now firmly established. We
believe that there are three fundamental particles the neutron,
12 Introduction
the proton, and the electron. A complete treatise would also
discuss the positron, which we have mentioned, the neutrino and
the mesotron. The deuteron and alpha particle, which have
already been mentioned, are important complex particles.
1.24. According to our present views the nuclei of all atomic
species are made up of neutrons and protons. The number of
protons is equal to the atomic number, Z. The number of neu-
trons, N, is equal to the difference between the mass number and
the atomic number, or A Z. There are two sets of forces
acting on these particles, ordinary electric coulomb forces of
repulsion between the positive charges and very short-range
forces between all the particles. These last forces are only
partly understood, and we shall not attempt to discuss them.
Suffice it to say that combined effects of these attractive and
repulsive forces are such that only certain combinations of
neutrons and protons are stable. If the neutrons and protons are
few in number, stability occurs when their numbers are about
equal. For larger nuclei, the proportion of neutrons required for
stability is greater. Finally, at the end of the periodic table,
where the number of protons is over 90 and the number of
neutrons nearly 1 50, there are no completely stable nuclei. (Some
of the heavy nuclei are almost stable as evidenced by very long
half-lives.) If an unstable nucleus is formed artificially by adding
an extra neutron or proton, eventually a change to a stable form
occurs. Strangely enough, this is not accomplished by ejecting a
proton or a neutron but by ejecting a positron or an electron;
apparently within the nucleus a proton converts itself into a
neutron and positron (or a neutron converts itself into a proton
and electron), and the light charged particle is ejected. In other
words, the mass number remains the same but the atomic number
changes. The stability conditions are not very critical so that for
a given mass number, i.e., given total number of protons and
neutrons, there may be several stable arrangements of protons
and neutrons (at most three or five) giving several isobars. For a
given atomic number, i.e., given number of protons, conditions
can vary still more widely so that some of the heavy elements
Introduction 13
have as many as ten or twelve stable isotopes. Some two hundred
and fifty different stable nuclei have been identified, ranging in
mass number from one to two hundred and thirty-eight and in
atomic number from one to ninety-two.
1.25. All the statements we have been making are based on
experimental evidence. The theory of nuclear forces is still
incomplete, but it has been developed on quantum-mechanical
principles sufficiently to explain not only the above observations
but more detailed empirical data on artificial radioactivity and
on differences between nuclei with odd and even mass numbers.
ARTIFICIAL RADIOACTIVITY
1.26. We mentioned the emission of positrons or electrons
by nuclei seeking stability. Electron emission (beta rays) was
already familiar in the study of naturally radioactive sub-
stances, but positron emission was not found in the case of such
substances. In fact, the general discussion presented above
obviously was based in part on information that cannot be pre-
sented in this report. We shall, however, give a brief account of
the discovery of "artificial" radioactivity and what is now known
about it.
1.27. In 1934, Curie and Joliot reported that certain light
elements (boron, magnesium, aluminum) which had been bom-
barded with alpha particles continued to emit positrons for some
time after the bombardment was stopped. In other words,
alpha-particle bombardment produced radioactive forms of
boron, magnesium, and aluminum. Curie and Joliot actually
measured half-lives of 14 minutes, 2.5 minutes, and 3.25 minutes,
respectively, for the radioactive substances formed by the alpha-
particle bombardment.
1.28. This result stimulated similar experiments all over the
world. In particular, E. Fermi reasoned that neutrons, because
of their lack of charge, should be effective in penetrating nuclei,
especially those of high atomic number which repel protons and
alpha particles strongly. He was able to verify his prediction
almost immediately, finding that the nucleus of the bombarded
14 Introduction
atom captured the neutron and that there was thus produced an
unstable nucleus which then achieved stability by emitting an
electron. Thus, the final, stable nucleus was one unit higher in
mass number and one unit higher in atomic number than the
initial target nucleus.
1.29. As a result of innumerable experiments carried out since
1934, radioactive isotopes of nearly every element in the periodic
table can now be produced. Some of them revert to stability by
the emission of positrons, some by the emission of electrons,
some by a process known as K-electron capture which we shall
not discuss, and a small number (probably three) by alpha-
particle emission. Altogether some five hundred unstable nuclear
species have been observed, and in most cases their atomic
numbers and mass numbers have been identified.
1.30. Not only do these artificially radioactive elements play
an important role throughout the project with which we are
concerned, but their future value in medicine, in "tracer"
chemistry, and in many other fields of research can hardly be
overestimated.
ENERGY CONSIDERATIONS
NUCLEAR BINDING ENERGIES
1.31. In describing radioactivity and atomic structure we have
deliberately avoided quantitative data and have not mentioned
any applications of the equivalence of mass and energy which we
announced as the guiding principle of this report. Now when
we must speak of quantitative details, not merely of general
principles.
1.32. We have spoken of stable and unstable nuclei made up
of assemblages of protons and neutrons held together by nuclear
forces. It is a general principle of physics that work must be
done on a stable system to break it up. Thus, if an assemblage of
neutrons and protons is stable, energy must be supplied to sepa-
rate its constituent particles. If energy and mass are really
equivalent, then the total mass of a stable nucleus should be less
Introduction 1 5
than the total mass of the separate protons and neutrons that
go to make it up. This mass difference, then, should be equivalent
to the energy required to disrupt the nucleus completely, which
is called the binding energy. Remember that the masses of all
nuclei were "approximately" whole numbers. It is the small
differences from whole numbers that are significant.
1.33. Consider the alpha particle as an example. It is stable;
since its mass number is four and its atomic number two it con-
sists of two protons and two neutrons. The mass of a proton is
1.00758 and that of a neutron is 1.00893 (see Appendix 2), so
that the total mass of the separate components of the helium
nucleus is
2 X 1.00758 -f 2 X 1.00893 = 4.03302
whereas the mass of the helium nucleus itself is 4.00280. Neglect-
ing the last two decimal places we have 4.033 and 4.003, a
difference of 0.030 mass units. This, then, represents the "binding
energy" of the protons and neutrons in the helium nucleus. It
looks small, but recalling Einstein's equation, E = me 2 , we re-
member that a small amount of mass is equivalent to a large
amount of energy. Actually 0.030 mass units is equal to 4. 5 X 10~ 5
ergs per nucleus or 2.7 X 10 19 ergs per gram molecule of helium.
In units more familiar to the engineer or chemist, this means that
to break up the nuclei of all the helium atoms in a gram of helium
would require 1.62 X 10 11 gram calories or 190,000 kilowatt
hours of energy. Conversely, if free protons and neutrons could
be assembled into helium nuclei, this energy would be released.
1.34. Evidently it is worth exploring the possibility of getting
energy by combining protons and neutrons or by transmuting
one kind of nucleus into another. Let us begin by reviewing
present-day knowledge of the binding energies of various nuclei.
MASS SPECTRA AND BINDING ENERGIES
1.35. Chemical atomic-weight determinations give the average
weight of a large number of atoms of a given element. Unless the
element has only one isotope, the chemical atomic weight is not
1 6 Introduction
proportional to the mass of individual atoms. The mass spectro-
graph developed by F. W. Aston and others from the earlier
apparatus of J. J. Thomson measures the masses of individual
isotopes. Indeed, it was just such measurements that proved the
existence of isotopes and showed that on the atomic-weight scale
the masses of all atomic species were very nearly whole numbers.
These whole numbers, discovered experimentally, are the mass
numbers which we have already defined and which represent
the sums of the numbers of the protons and neutrons; their
discovery contributed largely to our present views that all nuclei
are combinations of neutrons and protons.
1.36. Improved mass spectrograph data supplemented in a
few cases by nuclear reaction data have given accurate figures
for binding energies for many atomic species over the whole
range of atomic masses. This binding energy, B, is the difference
between the true nuclear mass, M, and the sum of the masses
of all the protons and neutrons in the nucleus. That is,
B = (ZM P + NM n ) - M
where M p and M n are the masses of the proton and neutron
respectively, Z is the number of protons, N = A Z is the num-
ber of neutrons, and M is the true mass of the nucleus. It is more
interesting to study the binding energy per particle, B/A, than
B itself. Such a study shows that, apart from fluctuations in the
light nuclei, the general trend of the binding energy per particle
is to increase rapidly to a flat maximum around A = 60 (nickel)
and then decrease again gradually. Evidently the nuclei in the
middle of the periodic table nuclei of mass numbers 40 to 100
are the most strongly bound. Any nuclear reaction where the
particles in the resultant nuclei are more strongly bound than
the particles in the initial nuclei will release energy. Speaking in
thermochemical terms, such reactions are exothermic. Thus, in
general, energy may be gained by combining light nuclei to
form heavier ones or by breaking very heavy ones into two or
three smaller fragments. Also, there are a number of special
cases of exothermic nuclear disintegrations among the first ten
Introduction 1 7
or twelve elements of the periodic table, where the binding
energy per particle varies irregularly from one element to another.
1.37. So far we seem to be piling one supposition on another.
First we assumed that mass and energy were equivalent; now we
are assuming that atomic nuclei can be rearranged with a
consequent reduction in their total mass, thereby releasing energy
which can then be put to use. It is time to talk about some ex-
periments that convinced physicists of the truth of these statements.
EXPERIMENTAL PROOF OF THE EQUIVALENCE
OF MASS AND ENERGY
1.38. As we have already said, Rutherford's work in 1919 on
artificial nuclear disintegration was followed by many similar
experiments. Gradual improvement in high-voltage technique
made it possible to substitute artificially produced high-speed
ions of hydrogen or helium for natural alpha particles. J. D.
Cockcroft and E. T. S. Walton in Rutherford's laboratory were
the first to succeed in producing nuclear changes by such methods.
In 1932 they bombarded a target of lithium with protons of 700
kilovolts energy and found that alpha particles were ejected from
the target as a result of the bombardment. The nuclear reaction
which occurred can be written symbolically as
3 Li 7 + iH 1 - 2 He 4 + 2 He 4
where the subscript represents the positive charge on the nucleus
(atomic number) and the superscript is the number of massive
particles in the nucleus (mass number). As in a chemical equa-
tion, quantities on the left must add up to those on the right; thus
the subscripts total four and the superscripts eight on each side.
1.39. Neither mass nor energy has been included in this equa-
tion. In general, the incident proton and the resultant alpha
particles will each have kinetic energy. Also, the mass of two
alpha particles will not be precisely the same as the sum of the
masses of a proton and a lithium atom. According to our theory,
the totals of mass and energy taken together should be the same
before and after the reaction. The masses were known from mass
1 8 Introduction
spectra. On the left (Li 7 -f H 1 ) they totalled 8.0241, on the right
(2He 4 ) 8.0056, so that 0.0185 units of mass had disappeared in
the reaction. The experimentally determined energies of the
alpha particles were approximately 8.5 million electron volts
each, a figure compared to which the kinetic energy of the inci-
dent proton could be neglected. Thus 0.0185 units of mass had
disappeared and 17 Mev of kinetic energy had appeared. Now
0.0185 units of mass is 3.07 X 10~ 26 grams, 17 Mev is 27.2 X 10~ 6
ergs and c is 3 X 10 10 cm/sec. (See Appendix 2.) If we substitute
these figures into Einstein's equation, E = me 2 , on the left side
we have 27.2 X 10~ 6 ergs and on the right side we have 27.6 X
10~ 6 ergs, so that the equation is found to be satisfied to a good
approximation. In other words, these experimental results prove
that the equivalence of mass and energy was correctly stated by
Einstein.
NUCLEAR REACTIONS
METHODS OF NUCLEAR BOMBARDMENT
1.40. Cockcroft and Walton produced protons of fairly high
energy by ionizing gaseous hydrogen and then accelerating the
ions in a transformer-rectifier high-voltage apparatus. A similar
procedure can be used to produce high-energy deuterons from
deuterium or high-energy alpha particles from helium. Higher
energies can be attained by accelerating the ions in cyclotrons
or Van de Graaff machines. However, to obtain high-energy
gamma radiation or most important of all to obtain neu-
trons, nuclear reactions themselves must be used as sources.
Radiations of sufficiently high energy come from certain naturally
radioactive materials or from certain bombardments. Neutrons
are commonly produced by the bombardment of certain elements,
notably beryllium or boron, by natural alpha particles, or by
bombarding suitable targets with protons or deuterons. The most
common source of neutrons is a mixture of radium and beryllium
where the alpha particles from radium and its decay products
penetrate the Be 9 nuclei, which then give off neutrons and become
Introduction 1 9
stable C 12 nuclei (ordinary carbon). A frequently used "beam"
source of neutrons results from accelerated deuterons impinging
on "heavy water" ice. Here the high-speed deuterons strike the
target deuterons to produce neutrons and He 3 nuclei. Half a
dozen other reactions are also used involving deuterium, lithium,
beryllium, or boron as targets. Note that in all these reactions the
total mass number and total charge number are unchanged.
1.41. To summarize, the agents that are found to initiate
nuclear reactions are in approximate order of importance
neutrons, deuterons, protons, alpha particles, gamma rays and,
rarely, heavier particles.
RESULTS OF NUCLEAR BOMBARDMENT
1.42. Most atomic nuclei can be penetrated by at least one
type of atomic projectile (or by gamma radiation). Any such
penetration may result in a nuclear rearrangement in the course
of which a fundamental particle is ejected or radiation is emitted
or both. The resulting nucleus may be one of the naturally avail-
able stable species, or more likely it may be an atom of a
different type which is radioactive, eventually changing to still a
different nucleus. This may in turn be radioactive and, if so, will
again decay. The process continues until all nuclei have changed
to a stable type. There are two respects in which these artificially
radioactive substances differ from the natural ones: many of
them change by emitting positrons (unknown in natural radio-
activity) and very few of them emit alpha particles. In every one
of the cases where accurate measurements have been made, the
equivalence of mass and energy has been demonstrated and the
mass-energy total has remained constant. (Sometimes it is neces-
sary to invoke neutrinos to preserve mass-energy conservation.)
NOTATION
1.43. A complete description of a nuclear reaction should
include the nature, mass and energy of the incident particle, also
the nature (mass number and atomic number), mass and energy
(usually zero) of the target particle, also the nature, mass and
20 Introduction
energy of the ejected particles (or radiation), and finally the
nature, mass and energy of the remainder. But all of these are
rarely known and for many purposes their complete specification
is unnecessary. A nuclear reaction is frequently described by a
notation that designates first the target by chemical symbol and
mass number if known, then the projectile, then the emitted
particle, and then the remainder. In this scheme the neutron
is represented by the letter n, the proton by p, the deuteron by d,
the alpha particle by <x, and the gamma ray by 7. Thus the
radium-beryllium neutron reaction can be written Be 2 (a, n)C 12
and the deuteron-deuteron reaction H 2 (d, n)He 3 .
TYPES OF REACTION
1.44. Considering the five different particles (n, p. d, a, 7)
both as projectiles and emitted products, we might expect to
find twenty-five combinations possible. Actually the deuteron
very rarely occurs as a product particle, and the photon initiates
only two types of reaction. There are, however, a few other types
of reaction, such as (n, 2n), (d, H 3 ), and fission, which bring the
total known types to about twenty-five. Perhaps the (n, 7) reac-
tion should be specifically mentioned as it is very important in
one process which will concern us. It is often called "radiative
capture" since the neutron remains in the nucleus and only a
gamma ray comes out.
PROBABILITY AND CROSS SECTION
1.45. So far nothing has been said about the probability of
nuclear reactions. Actually it varies widely. There is no guarantee
that a neutron or proton headed straight for a nucleus will
penetrate it at all. It depends on the nucleus and on the incident
particle. In nuclear physics, it is found convenient to express
probability of a particular event by a "cross section." Statistically,
the centers of the atoms in a thin foil can be considered as points
evenly distributed over a plane. The center of an atomic projec-
tile striking this plane has geometrically a definite probability of
Introduction 21
passing within a certain distance (r) of one of these points. In fact,
if there are n atomic centers in an area A of the plane, this prob-
ability is nrr 2 /A, which is simply the ratio of the aggregate area
of circles of radius r drawn around the points to the whole area.
If we think of the atoms as impenetrable steel discs and the
impinging particle as a bullet of negligible diameter, this ratio
is the probability that the bullet will strike a steel disc, i.e., that
the atomic projectile will be stopped by the foil. If it is the fraction
of impinging atoms getting through the foil which is measured, the
result can still be expressed in terms of the equivalent stopping
cross section of the atoms. This notion can be extended to any
interaction between the impinging particle and the atoms in the
target. For example, the probability that an alpha particle strik-
ing a beryllium target will produce a neutron can be expressed
as the equivalent cross section of beryllium for this type of
reaction.
1.46. In nuclear physics it is conventional to consider that the impinging
particles have negligible diameter. The technical definition of cross section
for any nuclear process is therefore:
number of
processes occurring
, 7- = (number of target nuclei per cm 2 ) X (nuclear
incident particles CTOSS scction in cm2)
It should be noted that this definition is for the cross section per nucleus.
Gross sections can be computed for any sort of process, such as capture
scattering, production of neutrons, etc. In many cases, the number of parti-
cles emitted or scattered in nuclear processes is not measured directly; one
merely measures the attenuation produced in a parallel beam of incident
particles by the interposition of a known thickness of a particular material.
The cross section obtained in this way is called the total cross section and is
usually denoted by <r.
1.47. As indicated in paragraph 1.11, the typical nuclear diameter is of
the order of 10~ 12 cm. We might therefore expect the cross sections for nuclear
reactions to be of the order of xd 2 /4 or roughly 10~ 24 cm 2 and this is the unit
in which they are usually expressed. Actually the observed cross sections
vary enormously. Thus for slow neutrons absorbed by the (n, 7) reaction the
cross section in some cases is as much as 1,000 X 10~ 24 cm 2 , while the cross
sections for transmutations by gamma-ray absorption are in the neighbor-
hood of (1/1,000) X 10~ 24 cm 2 .
22 Introduction
PRACTICABILITY OF ATOMIC POWER
IN 1939
SMALL SCALE OF EXPERIMENTS
1.48. We have talked glibly about the equivalence of mass
and energy and about nuclear reactions, such as that of protons
on lithium, where energy was released in relatively large amounts.
Now let us ask why atomic power plants did not spring up all
over the world in the 'thirties. After all, if we can get 2.76 X 10~ 5
ergs from an atom of lithium struck by a proton, we might expect
to obtain approximately half a million kilowatt hours by com-
bining a gram of hydrogen with seven grams of lithium. It looks
better than burning coal. The difficulties are in producing the
high-speed protons and in controlling the energy produced. All
the experiments we have been talking about were done with
very small quantities of material, large enough in numbers of
atoms, to be sure, but in terms of ordinary masses infinitesimal
not tons or pounds or grams, but fractions of micrograms. The
amount of energy used up in the experiment was always far
greater than the amount generated by the nuclear reaction.
1.49. Neutrons are particularly effective in producing nuclear
disintegration. Why weren't they used? If their initial source was
an ion beam striking a target, the limitations discussed in the last
paragraph applied. If a radium and beryllium source was to be
used, the scarcity of radium was a difficulty.
THE NEED OF A CHAIN REACTION
1.50. Our common sources of power, other than sunlight and
water power, are chemical reactions usually the combustion of
coal or oil. They release energy as the result of rearrangements
of the outer electronic structures of the atoms, the same kind of
process that supplies energy to our bodies. Combustion is always
self-propagating; thus lighting a fire with a match releases enough
heat to ignite the neighboring fuel, which releases more heat which
ignites more fuel, and so on. In the nuclear reactions we have
Introduction 23
described this is not generally true; neither the energy released
nor the new particles formed are sufficient to maintain the reac-
tion. But we can imagine nuclear reactions emitting particles of
the same sort that initiate them and in sufficient numbers to
propagate the reaction in neighboring nuclei. Such a self-
propagating reaction is called a "chain reaction" and such con-
ditions must be achieved if the energy of the nuclear reactions
with which we are concerned is to be put to large-scale use.
PERIOD OF SPECULATION
1.51. Although there were no atomic power plants built in
the 'thirties, there were plenty of discoveries in nuclear physics
and plenty of speculation. A theory was advanced by H. Bethe
to explain the heat of the sun by a cycle of nuclear changes
involving carbon, hydrogen, nitrogen, and oxygen, and leading
eventually to the formation of helium.* This theory is now
generally accepted. The discovery of a few (n, 2n) nuclear reac-
tions (i.e., neutron-produced and neutron-producing reactions)
suggested that a self-multiplying chain reaction might be initiated
under the right conditions. There was much talk of atomic power
and some talk of atomic bombs. But the last great step in this
preliminary period came after four years of stumbling. The
effects of neutron bombardment of uranium, the most complex
element known, had been studied by some of the ablest physicists.
The results were striking but confusing. The story of their gradual
interpretation is intricate and highly technical, a fascinating tale
of theory and experiment. Passing by the earlier inadequate
* The series of reactions postulated was
(1) eC 1J + iH*-+ 7 Ni 3
(2) 7 N" -* 6 C 13 + 1C
(3) 6 C 13 +'iHi-> 7 N"
(4) TN^ + iH 1 -*^ 15
(5) 8 15 -> T N + ie
(6) 7 N 15 + jH l - eG" + 2 He
The net effect is the transformation of hydrogen into helium and positrons
(designated as ie) and the release of about thirty million electron volts
energy.
24 Introduction
explanations, we shall go directly to the final explanation, which,
as so often happens, is relatively simple.
DISCOVERY OF URANIUM FISSION
1.52. As has already been mentioned, the neutron proved to
be the most effective particle for inducing nuclear changes.
This was particularly true for the elements of highest atomic
number and weight where the large nuclear charge exerts
strong repulsive forces on deuteron or proton projectiles but not
on uncharged neutrons. The results of the bombardment of
uranium by neutrons had proved interesting and puzzling. First
studied by Fermi and his colleagues in 1934, they were not
properly interpreted until several years later.
1.53. On January 16, 1939, Niels Bohr of Copenhagen, Den-
mark, arrived in this country to spend several months in Prince-
ton, N. J., and was particularly anxious to discuss some abstract
problems with Einstein. (Four years later Bohr was to escape
from Nazi-occupied Denmark in a small boat.) Just before Bohr
left Denmark two of his colleagues, O. R. Frisch and L. Meitner
(both refugees from Germany), had told him their guess that the
absorption of a neutron by a uranium nucleus sometimes caused
that nucleus to split into approximately equal parts with the
release of enormous quantities of energy, a process that soon
began to be called nuclear "fission." The occasion for this
hypothesis was the important discovery of O. Hahn and F.
Strassmann in Germany (published in Naturwissenschaften in early
January 1939) which proved that an isotope of barium was
produced by neutron bombardment of uranium. Immediately
on arrival in the United States Bohr communicated this idea to
his former student J. A. Wheeler and others at Princeton, and
from them the news spread by word of mouth to neighboring
physicists including E. Fermi at Columbia University. As a result
of conversations among Fermi, J. R. Dunning, and G. B.
Pegram, a search was undertaken at Columbia for the heavy
pulses of ionization that would be expected from the flying
fragments of the uranium nucleus. On January 26, 1939, there
Introduction 25
was a conference on theoretical physics at Washington, D. C.,
sponsored jointly by the George Washington University and the
Carnegie Institution of Washington. Fermi left New York to
attend this meeting before the Columbia fission experiments had
been tried. At the meeting Bohr and Fermi discussed the problem
of fission, and in particular Fermi mentioned the possibility that
neutrons might be emitted during the process. Although this
was only a guess, its implication of the possibility of a chain
reaction was obvious. A number of sensational articles were pub-
FISSION FRAGMENT
>NE TO THREE NEUTRONS
NEUTRON "^
FISSION FRAGMENT
lished in the press on this subject. Before the meeting in Washing-
ton was over, several other experiments to confirm fission had been
initiated, and positive experimental confirmation was reported
from four laboratories (Columbia University, Carnegie Institu-
tion of Washington, Johns Hopkins University, University of
California) in the February 15, 1939, issue of the Physical Review.
By this time Bohr had heard that similar experiments had been
made in his laboratory in Copenhagen about January 15. (Letter
by Frisch to Nature dated January 16, 1939, and appearing in the
February 18 issue.) F. Joliot in Paris had also published his first
results in the Comptes Rendus of January 30, 1939. From this time
on there was a steady flow of papers on the subject of fission, so
that by the time (December 6. 1939) L. A. Turner of Princeton
26 Introduction
wrote a review article on the subject in the Reviews of Modern
Physics nearly one hundred papers had appeared. Complete
analysis and discussion of these papers have appeared in Turner's
article and elsewhere.
GENERAL DISCUSSION OF FISSION
1 .54. Consider the suggestion of Frisch and Meitner in the light
of the two general trends that had been discovered in nuclear
structure: first, that the proportion of neutrons goes up with
atomic number; second, that the binding energy per particle
is a maximum for the nuclei of intermediate atomic number.
Suppose the U-238 nucleus is broken exactly in half; then,
neglecting the mass of the incident neutron, we have two nuclei
of atomic number 46 and mass number 119. But the heaviest
stable isotope of palladium (Z = 46) has a mass number of only
110. Therefore to reach stability each of these imaginary new
nuclei must eject nine neutrons, becoming 4ePd 110 nuclei; or
four neutrons in each nucleus must convert themselves to protons
by emitting electrons, thereby forming stable tin nuclei of mass
number 119 and atomic number 50; or a combination of such
ejections and conversions must occur to give some other pair of
stable nuclei. Actually, as was suggested by Hahn and Strass-
mann's identification of barium (Z = 56, A = 135 to 140) as a
product of fission, the split occurs in such a way as to produce
two unequal parts of mass numbers about 140 and 90 with the
emission of a few neutrons and subsequent radioactive decay by
electron emission until stable nuclei are formed. Calculations
from binding-energy data show that any such rearrangement
gives an aggregate resulting mass considerably less than the
initial mass of the uranium nucleus, and thus that a great deal
of energy must be released.
1.55. Evidently, there were three major implications of the
phenomenon of fission: the release of energy, the production of
radioactive atomic species and the possibility of a neutron chain
reaction. The energy release might reveal itself in kinetic energy
of the fission fragments and in the subsequent radioactive dis-
Introduction 27
integration of the products. The possibility of a neutron chain
reaction depended on whether neutrons were in fact emitted
a possibility which required investigation.
1.56. These were the problems suggested by the discovery of
fission, the kind of problem reported in the journals in 1939 and
1940 and since then investigated largely in secret. The study of
the fission process itself, including production of neutrons and
fast fragments, has been largely carried out by physicists using
counters, cloud chambers, etc. The study and identification of
the fission products has been carried out largely by chemists,
who have had to perform chemical separations rapidly even with
submicroscopic quantities of material and to make repeated
determinations of the half-lives of unstable isotopes. We shall
summarize the state of knowledge as of June 1 940. By that time
the principal facts about fission had been discovered and revealed
to the scientific world. A chain reaction had not been obtained,
but its possibility at least in principle was clear and several
paths that might lead to it had been suggested.
STATE OF KNOWLEDGE IN JUNE 1940
DEFINITE AND GENERALLY KNOWN INFORMATION ON FISSION
1.57. All the following information was generally known in
June 1940, both here and abroad:
(1) That three elements uranium, thorium, and protoactinium
when bombarded by neutrons sometimes split into approxi-
mately equal fragments, and that these fragments were isotopes
of elements in the middle of the periodic table, ranging from
selenium (Z = 34) to lanthanum (Z = 57).
(2) That most of these fission fragments were unstable, decaying
radioactively by successive emission of beta particles through a
series of elements to various stable forms.
(3) That these fission fragments had very great kinetic energy.
(4) That fission of thorium and protoactinum was caused only
28 Introduction
by fast neutrons (velocities of the order of thousands of miles
per second).
(5) That fission in uranium could be produced by fast or slow
(so-called thermal velocity) neutrons; specifically, that thermal
neutrons caused fission in one isotope, U-235, but not in the
other, U-238, and that fast neutrons had a lower probability of
causing fission in U-235 than thermal neutrons.
(6) That at certain neutron speeds there was a large capture
cross section in U-238 producing U-239 but not fission.
(7) That the energy released per fission of a uranium nucleus
was approximately 200 million electron volts.
(8) That high-speed neutrons were emitted in the process of
fission.
(9) That the average number of neutrons released per fission
was somewhere between one and three.
(10) That high-speed neutrons could lose energy by inelastic
collision with uranium nuclei without any nuclear reaction
taking place.
(11) That most of this information was consistent with the semi-
empirical theory of nuclear structure worked out by Bohr and
Wheeler and others; this suggested that predictions based on
this theory had a fair chance of success.
SUGGESTION OF PLUTONIUM FISSION
1.58. It was realized that radiative capture of neutrons by
U-238 would probably lead by two successive beta-ray emissions
to the formation of a nucleus for which Z = 94 and A = 239.
Consideration of the Bohr- Wheeler theory of fission and of certain
empirical relations among the nuclei by L. A. Turner and others
suggested that this nucleus would be a fairly stable alpha emitter
and would probably undergo fission when bombarded by thermal
neutrons. Later the importance of such thermal fission to the
maintenance of the chain reaction was foreshadowed in private
correspondence and discussion. In terms of our present knowledge
and notation the particular reaction suggested is as follows:
Introduction 29
where Np and Pu are the chemical symbols now used for the
two new elements, neptunium and plutonium; on 1 represents the
neutron, and _ie represents an ordinary (negative) electron.
Plutonium 239 is the nucleus rightly guessed to be fissionable by
thermal neutrons. It will be discussed fully in later chapters.
GENERAL STATE OF NUCLEAR PHYSICS
1.59. By 1940 nuclear reactions had been intensively studied
for over ten years. Several books and review articles on nuclear
physics had been published. New techniques had been developed
for producing and controlling nuclear projectiles, for studying
artificial radioactivity, and for separating submicroscopic quan-
tities of chemical elements produced by nuclear reactions. Isotope
masses had been measured accurately. Neutron-capture cross
sections had been measured. Methods of slowing down neutrons
had been developed. Physiological effects of neutrons had been
observed; they had even been tried in the treatment of cancer.
All such information was generally available; but it was very
incomplete. There were many gaps and many inaccuracies.
The techniques were difficult and the quantities of materials
available were often submicroscopic. Although the fundamental
principles were clear, the theory was full of unverified assump-
tions and calculations were hard to make. Predictions made in
1940 by different physicists of equally high ability were often
at variance. The subject was in all too many respects an art,
rather than a science.
SUMMARY
1.60. Looking back on the year 1940, we see that all the pre-
requisites to a serious attack on the problem of producing atomic
bombs and controlling atomic power were at hand. It had been
proved that mass and energy were equivalent. It had been proved
that the neutrons initiating fission of uranium reproduced them-
30 Introduction
selves in the process and that therefore a multiplying chain
reaction might occur with explosive force. To be sure, no one
knew whether the required conditions could be achieved, but
many scientists had clear ideas as to the problems involved and
the directions in which solutions might be sought. The next
chapter of this report gives a statement of the problems and
serves as a guide to the developments of the past five years.
CHAPTER II. STATEMENT OF THE PROBLEM
INTRODUCTION
2.1. From the time of the first discovery of the large amounts
of energy released in nuclear reactions to the time of the discovery
of uranium fission, the idea of atomic power or even atomic bombs
was discussed off and on in scientific circles. The discovery of
fission made this talk seem much less speculative, but realization
of atomic power still seemed in the distant future and there was
an instinctive feeling among many scientists that it might not,
in fact, ever be realized. During 1939 and 1940 many public
statements, some of them by responsible scientists, called atten-
tion to the enormous energy available in uranium for explosives
and for controlled power, so that U-235 became a familiar by-
word indicating great things to come. The possible military
importance of uranium fission was called to the attention of the
government (see Chapter III), and in a conference with repre-
sentatives of the Navy Department in March 1939 Fermi sug-
gested the possibility of achieving a controllable reaction using
slow neutrons or a reaction of an explosive character using fast
neutrons. He pointed out, however, that the data then available
might be insufficient for accurate predictions.
2.2. By the summer of 1940 it was possible to formulate the
problem fairly clearly, although it was still far from possible to
answer the various questions involved or even to decide whether
a chain reaction ever could be obtained. In this chapter we shall
give a statement of the problem in its entirety. For purposes of
clarification we may make use of some knowledge which actually
was not acquired until a later date.
THE CHAIN-REACTION PROBLEM
2.3. The principle of operation of an atomic bomb or power
plant utilizing uranium fission is simple enough. If one neutron
31
32 Statement of the Problem
causes a fission that produces more than one new neutron, the
number of fissions may increase tremendously with the release
of enormous amounts of energy. It is a question of probabilities.
Neutrons produced in the fission process may escape entirely
from the uranium, may be captured by uranium in a process not
resulting in fission, or may be captured by an impurity. Thus the
question of whether a chain reaction does or does not go depends
on the result of a competition among four processes:
(1) escape,
(2) non-fission capture by uranium,
(3) non-fission capture by impurities,
(4) fission capture.
If the loss of neutrons by the first three processes is less than the
surplus produced by the fourth, the chain reaction occurs; other-
wise it does not. Evidently any one of the first three processes
may have such a high probability in a given arrangement that
the extra neutrons created by fission will be insufficient to keep
the reaction going. For example, should it turn out that process
(2) non-fission capture by uranium has a much higher prob-
ability than fission capture, there would presumably be no
possibility of achieving a chain reaction.
2.4. An additional complication is that natural uranium con-
tains three isotopes: U-234, U-235, and U-238, present to the
extent of approximately 0.006, 0.7, and 99.3 percent, respectively.
We have already seen that the probabilities of processes (2)
and (4) are different for different isotopes. We have also seen
that the probabilities are different for neutrons of different
energies.
2.5. We shall now consider the limitations imposed by the
first three processes and how their effects can be minimized.
NEUTRON ESCAPE; CRITICAL SIZE
2.6. The relative number of neutrons which escape from a
quantity of uranium can be minimized by changing the size
Statement of the Problem 33
and shape. In a sphere any surface effect is proportional to the
square of the radius, and any volume effect is proportional to the
cube of the radius. Now the escape of neutrons from a quantity
of uranium is a surface effect depending on the area of the surface,
but fission capture occurs throughout the material and is there-
fore a volume effect. Consequently the greater the amount of
uranium, the less probable it is that neutron escape will pre-
dominate over fission capture and prevent a chain reaction.
Loss of neutrons by non-fission capture is a volume effect like
neutron production by fission capture, so that increase in size
makes no change in its relative importance.
2.7. The critical size of a device containing uranium is defined
as the size for which the production of free neutrons by fission
is just equal to their loss by escape and by non-fission capture.
In other words, if the size is smaller than critical, then by
definition no chain reaction will sustain itself. In principle it
was possible in 1940 to calculate the critical size, but in practice
the uncertainty of the constants involved was so great that the
various estimates differed widely. It seemed not improbable that
the critical size might be too large for practical purposes. Even
now estimates for untried arrangements vary somewhat from
time to time as new information becomes available.
USE OF A MODERATOR TO REDUCE NON-FISSION CAPTURE
2.8. In Chapter I we said that thermal neutrons have the
highest probability of producing fission of U-235 but we also
said that the neutrons emitted in the process of fission had high
speeds. Evidently it was an oversimplification to say that the
chain reaction might maintain itself if more neutrons were
created by fission than were absorbed. For the probability both
of fission capture and of non-fission capture depends on the speed
of the neutrons. Unfortunately, the speed at which non-fission
capture is most probable is intermediate between the average
speed of neutrons emitted in the fission process and the speed
at which fission capture is most probable.
34 Statement of the Problem
2.9. For some years before the discovery of fission, the cus-
tomary way of slowing down neutrons was to cause them to
pass through material of low atomic weight, such as hydrogenous
material. The process of slowing down or moderation is simply
one of elastic collisions between high-speed particles and particles
practically at rest. The more nearly identical the masses of
neutron and struck particle the greater the loss of kinetic energy
by the neutron. Therefore the light elements are most effective
as "moderators," i.e., slowing down agents, for neutrons.
2.10. It occurred to a number of physicists that it might be
possible to mix uranium with a moderator in such a way that
the high-speed fission neutrons, after being ejected from uranium
and before re-encountering uranium nuclei, would have their
speeds reduced below the speeds for which non-fission capture
is highly probable. Evidently the characteristics of a good moder-
ator are that it should be of low atomic weight and that it should
have little or no tendency to absorb neutrons. Lithium and boron
are excluded on the latter count. Helium is difficult to use because
it is a gas and forms no compounds. The choice of moderator
therefore lay among hydrogen, deuterium, beryllium, and
carbon. Even now no one of these substances can be excluded
from the list of practical possibilities. It was E. Fermi and L.
Szilard who proposed the use of graphite as a moderator for a
chain reaction.
USE OF A LATTICE TO REDUCE NON-FISSION CAPTURE
2.11. The general scheme of using a moderator mixed with
the uranium was pretty obvious. A specific manner of using a
moderator was first suggested in this country, so far as we can
discover, by Fermi and Szilard. The idea was to use lumps of
uranium of considerable size imbedded in a matrix of moderator
material. Such a lattice can be shown to have real advantages
over a homogeneous mixture. As the constants were more accu-
rately determined, it became possible to calculate theoretically
the type of lattice that would be most effective.
STRAY NEUTRON
ORIGINAL FISSION
FISSON FRAGMENT
FISSION FRAGMEN
ONE TO THREE
NEUTRONS FROM
FISSION PROCESS
A NEUTRON
SOMETIMES LOST
ONE NEW FISSION
ONE TO THREE
NEUTRONS AGAIN
TWO NEW FISSIONS
FISSION FRAGMENTS
CHANGES TO
PLUTONIUM
FISSION FRAGMENT
FISSION
FRAGMENT
FISSION
FRAGMENT
SEVERAL NEW NEUTRONS CAUSE MORE FISSIONS
SCHEMATIC DIAGRAM OF CHAIN REACTION FROM FISSION.NEGLECT1NG EFFECT
OF NEUTRON SPEED. IN AN EXPLOSIVE REACTION THE NUMBER OF
NEUTRONS MULTIPLIES INDEFINITELY. N A CONTROLLED REACTION
THE NUMBER OF NEUTRONS BUILDS UP TO A CERTAIN LEVEL AND THEN
REMAINS CONSTANT.
STRAY NEUTRON
FISSION FRAGMENT
/ PA
NEUT
ST\ <
RONS
3
FISSION f
BE LOST
| MODERATOR ]
/ SLOW NE
UTRONS \
\
MAY
CHANGES TO PLUTONIUM
FISSION FRAGMENT
FISSION FRAGMENT
/ I \
SLOW NEUTRONS TO CAUSE MORE FISSIONS
AND SO ON.
SCHEMATIC DIAGRAM OF FISSION CHAIN REACTION USING A
MODERATOR TO SLOW NEUTRONS TO SPEEDS MORE LIKELY TO
CAUSE FISSION
Statement of the Problem 35
REDUCTION OF NON-FISSION CAPTURE BY
ISOTOPE SEPARATION
2.12. In Chapter I it was stated that for neutrons of certain
intermediate speeds (corresponding to energies of a few electron
volts) U-238 has a large capture cross section for the production
of U-239 but not for fission. There is also a considerable prob-
ability of inelastic (i.e., non-capture-producing) collisions between
high-speed neutrons and U-238 nuclei. Thus the presence of the
U-238 tends both to reduce the speed of the fast neutrons and to
effect the capture of those of moderate speed. Although there
may be some non-fission capture by U-235, it is evident that
if we can separate the U-235 from the U-238 and discard the
U-238, we can reduce non-fission capture and can thus promote
the chain reaction. In fact, the probability of fission of U-235
by high-speed neutrons may be great enough to make the use
of a moderator unnecessary once the U-238 has been removed.
Unfortunately, U-235 is present in natural uranium only to the
extent of about one part in 140. Also, the relatively small
difference in mass between the two isotopes makes separation
difficult. In fact, in 1940 no large-scale separation of isotopes
had ever been achieved except for hydrogen, whose two isotopes
differ in mass by a factor of two. Nevertheless, the possibility of
separating U-235 was recognized early as being of the greatest
importance, and such separation has, in fact, been one of the
two major lines of Project effort during the past five years.
PRODUCTION AND PURIFICATION OF MATERIALS
2.13. It has been stated above that the cross section for capture
of neutrons varies greatly among different materials. In some it
is very high compared to the maximum fission cross section of
uranium. If, then, we are to hope to achieve a chain reaction,
we must reduce effect (3) non-fission capture by impurities
to the point where it is not serious. This means very careful
purification of the uranium metal and very careful purification
of the moderator. Calculations show that the maximum per-
36 Statement of the Problem
missible concentrations of many impurity elements are a few
parts per million in either the uranium or the moderator. When
it is recalled that up to 1 940 the total amount of uranium metal
produced in this country was not more than a few grams and
even this was of doubtful purity, that the total amount of metallic
beryllium produced in this country was not more than a few
pounds, that the total amount of concentrated deuterium pro-
duced was not more than a few pounds, and that carbon had
never been produced in quantity with anything like the purity
required of a moderator, it is clear that the problem of producing
and purifying materials was a major one.
CONTROL OF THE CHAIN REACTION
2.14. The problems that have been discussed so far have to
do merely with the realization of the chain reaction. If such a
reaction is going to be of use, we must be able to control it. The
problem of control is different depending on whether we are
interested in steady production of power or in an explosion. In
general, the steady production of atomic power requires a slow-
neutron-induced fission chain reaction occurring in a mixture
or lattice of uranium and moderator, while an atomic bomb
requires a fast-neutron-induced fission chain reaction in U-235
or Pu-239, although both slow- and fast-neutron fission may con-
tribute in each case. It seemed likely, even in 1940, that by using
neutron absorbers a power chain reaction could be controlled.
It was also considered likely, though not certain, that such a chain
reaction would be self-limiting by virtue of the lower probability
of fission-producing capture when a higher temperature was
reached. Nevertheless, there was a possibility that a chain-reacting
system might get out of control, and it therefore seemed necessary
to perform the chain-reaction experiment in an uninhabited
location.
PRACTICAL APPLICATION OF THE CHAIN REACTION
2.15. Up to this point we have been discussing how to produce
and control a nuclear chain reaction but not how to make use
Statement of the Problem 37
of it. The technological gap between producing a controlled
chain reaction and using it as a large-scale power source or an
explosive is comparable to the gap between the discovery of fire
and the manufacture of a steam locomotive.
2.16. Although production of power has never been the
principal object of this project, enough attention has been given
to the matter to reveal the major difficulty: the attainment of
high- temperature operation. An effective heat engine must not
only develop heat but must develop heat at a high temperature.
To run a chain-reacting system at a high temperature and to
convert the heat generated to useful work is very much more
difficult than to run a chain-reacting system at a low temperature.
2.17. Of course, the proof that a chain reaction is possible does
not itself insure that nuclear energy can be effective in a bomb.
To have an effective explosion it is necessary that the chain
reaction build up extremely rapidly; otherwise only a small
amount of the nuclear energy will be utilized before the bomb
flies apart and the reaction stops. It is also necessary that no
premature explosion occur. This entire "detonation" problem
was and still remains one of the most difficult problems in de-
signing a high-efficiency atomic bomb.
POSSIBILITY OF USING PLUTONIUM
2.18. So far, all our discussion has been primarily concerned
with the use of uranium itself. We have already mentioned the
suggestion that the element of atomic number 94 and mass 239,
commonly referred to as plutonium, might be very effective.
Actually, we now believe it to be of value comparable to pure
U-235. We have mentioned the difficulty of separating U-235
from the more abundant isotope U-238. These two isotopes are,
of course, chemically identical. But plutonium, although pro-
duced from U-238, is a different chemical element. Therefore,
if a process could be worked out for converting some of the U-238
to plutonium, a chemical separation of the plutonium from ura-
nium might prove more practicable than the isotopic separation
of U-235 from U-238.
38 Statement of the Problem
2.19. Suppose that we have set up a controllable chain reac-
tion in a lattice of natural uranium and a moderator say carbon,
in the form of graphite. Then as the chain reaction proceeds,
neutrons are emitted in the process of fission of the U-235 and
many of these neutrons are absorbed by U-238. This produces
U-239, each atom of which then emits a beta particle, becoming
neptunium (osNp 239 ). Neptunium, in turn, emits another beta
particle, becoming plutonium ( 9 4Pu 239 ), which emits an alpha
particle, decaying again to U-235, but so slowly that in effect
it is a stable element. (See figure on p. 8.) If, after the reaction
has been allowed to proceed for a considerable time, the mixture
of metals is removed, it may be possible to extract the plutonium
by chemical methods and purify it for use in a subsequent
fission chain reaction of an explosive nature.
COMBINED EFFECTS AND ENRICHED PILES
2.20. Three ways of increasing the likelihood of a chain
reaction have been mentioned: use of a moderator; attainment
of high purity of materials; use of special material, either U-235
or Pu. The three procedures are not mutually exclusive, and
many schemes have been proposed for using small amounts of
separated U-235 or Pu-239 in a lattice composed primarily of
ordinary uranium or uranium oxide and of a moderator or two
different moderators. Such proposed arrangements are usually
called "enriched piles."
USE OF THORIUM OR PROTOACTINIUM
OR OTHER MATERIAL
2.21. All our previous discussion has centered on the direct or
indirect use of uranium, but it was known that both thorium
and protoactinium also underwent fission when bombarded by
high-speed neutrons. The great advantage of uranium, at least
for preliminary work, was its susceptibility to slow neutrons.
There was not very much consideration given to the other two
substances. Protoactinium can be eliminated because of its
Statement of the Problem 39
scarcity in nature. Thorium is relatively plentiful but has no
apparent advantage over uranium.
2.22. It is not to be forgotten that theoretically many nuclear
reactions might be used to release energy. At present we see no
way of initiating or controlling reactions other than those
involving fission, but some such synthesis as has already been
mentioned as a source of solar energy may eventually be pro-
duced in the laboratory.
AMOUNTS OF MATERIALS NEEDED
2.23. Obviously it was impossible in the summer of 1940 to
make more than guesses as to what amounts of materials would
be needed to produce:
(1) a chain reaction with use of a moderator:
(2) a chain-reaction bomb in pure, or at least enriched, U-235
or plutonium.
A figure of one to one hundred kilograms of U-235 was com-
monly given at this time for the critical size of a bomb. This
would, of course, have to be separated from at least 140 times
as much natural uranium. For a slow-neutron chain reaction
using a moderator and unseparated uranium it was almost
certain that tons of metal and of moderator would be required.
AVAILABILITY OF MATERIALS
2.24. Estimates of the composition of the earth's crust show
uranium and thorium both present in considerable quantities
(about 4 parts per million of uranium and 12 parts per million
of thorium in the earth's crust). Deposits of uranium ore are
known to exist in Colorado, in the Great Bear Lake region of
northern Canada, in Joachimstal in Czechoslovakia, and in the
Belgian Congo. Many other deposits of uranium ore are known,
but their extent is in many cases unexplored. Uranium is always
found with radium although in much larger quantity. Both are
often found with vanadium ores. Small quantities of uranium
oxide have been used for many years in the ceramics industry.
2.25. Thorium is also rather widely distributed, occurring as
40 Statement of the Problem
thorium oxide in fairly high concentration in monazite sands.
Such sands are found to some extent in this country but particu-
larly in Brazil and in British India.
2.26. Early rough estimates, which are probably optimistic,
were that the nuclear energy available in known deposits of
uranium was adequate to supply the total power needs of this
country for 200 years (assuming utilization of U-238 as well as
U-235).
2.27. As has already been mentioned, little or no uranium
metal had been produced up to 1940 and information was so
scant that even the melting point was not known. (For example,
the Handbook of Physics and Chemistry for 1 943-1 944 says only that
the melting point is below 1850 C. whereas we now know it
to be in the neighborhood of 1150.) Evidently, as far as uranium
was concerned, there was no insurmountable difficulty as regards
obtaining raw materials or producing the metal, but there were
very grave questions as to how long it would take and how much
it would cost to produce the necessary quantities of pure metal.
2.28. Of the materials mentioned above as being suitable for
moderators, deuterium had the most obvious advantages. It is
present in ordinary hydrogen to the extent of about one part in
5,000. By 1940 a number of different methods for separating it
from hydrogen had been developed, and a few liters had been
produced in this country for experimental purposes. The only
large-scale production had been in a Norwegian plant, from
which several hundred liters of heavy water (DzO, deuterium
oxide) had come. As in the case of uranium, the problem was
one of cost and time.
2.29. Beryllium in the form of beryllium silicates is widely
found but only in small quantities of ore. Its use as an alloying
agent has become general in the last few years; for such use,
however, it is not necessary to produce the beryllium in metallic
form. In 1940 only 700 pounds of the metal were produced in
this country.
2.30. As far as carbon was concerned, the situation was
obviously quite different. There were many hundreds of tons
Statement of the Problem 41
of graphite produced every year in this country. This was one
of the reasons why graphite looked very desirable as a moderator.
The difficulties lay in obtaining sufficient quantities of graphite
of the required purity, particularly in view of the expanding
needs of war industry.
TIME AND COST ESTIMATES
2.31. Requirements of time and money depended not only
on many unknown scientific and technological factors but also
on policy decisions. Evidently years of time and millions of dollars
might be required to achieve the ultimate objective. About all
that was attempted at this time was the making of estimates as
to how long it would take and how much it would cost to clarify
the scientific and technological prospects. It looked as if it would
not be a very great undertaking to carry along the development
of the thermal-neutron chain reaction in a graphite-uranium
lattice to the point of finding out whether the reaction would
in fact go. Estimates made at the time were that approximately
a year and $100,000 would be required to get an answer. These
estimates applied to a chain-reacting system of very low power
without a cooling system or any means for using the energy
released.
HEALTH HAZARDS
2.32. It had been known for a long time that radioactive
materials were dangerous. They give off very penetrating radia-
tions gamma rays which are much like X-rays in their phy-
siological effects. They also give off beta and alpha rays which,
although less penetrating, can still be dangerous. The amounts
of radium used in hospitals and in ordinary physical measure-
ments usually comprise but a few milligrams. The amounts of
radioactive material produced by the fission of uranium in a
relatively small chain-reacting system may be equivalent to
hundreds or thousands of grams of radium. A chain-reacting
system also gives off intense neutron radiation known to be
42 Statement of the Problem
comparable to gamma rays as regards health hazards. Quite
apart from its radioactive properties, uranium is poisonous
chemically. Thus, nearly all work in this field is hazardous
particularly work on chain reactions and the resulting radio-
active products.
METHOD OF APPROACH TO THE PROBLEM
2.33. There were two ways of attacking the problem. One was
to conduct elaborate series of accurate physical measurements on
absorption cross sections of various materials for various neutron-
induced processes and various neutron energies. Once such data
were available, calculations as to what might be done in the
way of a chain reaction could be made with fair accuracy. The
other approach was the purely empirical one of mixing uranium
or uranium compounds in various ways with various moderators
and observing what happened. Similar extremes of method were
possible in the case of the isotope-separation problem. Actually
an intermediate or compromise approach was adopted in both
cases.
POWER VS. BOMB
2.34. The expected military advantages of uranium bombs
were far more spectacular than those of a uranium power plant.
It was conceivable that a few uranium bombs might be decisive
in winning the war for the side first putting them into use. Such
thoughts were very much in the minds of those working in this
field, but the attainment of a slow-neutron chain reaction
seemed a necessary preliminary step in the development of our
knowledge and became the first objective of the group interested
in the problem. This also seemed an important step in con-
vincing military authorities and the more skeptical scientists
that the whole notion was not a pipe dream. Partly for these
reasons and partly because of the extreme secrecy imposed about
this time, the idea of an atomic bomb does not appear much in
the records between the summer of 1940 and the fall of 1941.
Statement of the Problem 43
MILITARY USEFULNESS
2.35. If all the atoms in a kilogram of U-235 undergo fission,
the energy released is equivalent to the energy released in the
explosion of about 20,000 short tons of TNT. If the critical size
of a bomb turns out to be practical say, in the range of one to
one hundred kilograms and all the other problems can be
solved, there remain two questions. First, how large a percentage
of the fissionable nuclei can be made to undergo fission before
the reaction stops; i.e., what is the efficiency of the explosion?
Second, what is the effect of so concentrated a release of energy?
Even if only 1 per cent of the theoretically available energy is
released, the explosion will still be of a totally different order of
magnitude from that produced by any previously known type
of bomb. The value of such a bomb was thus a question for
military experts to consider very carefully.
SUMMARY
2.36. It had been established (1) that uranium fission did
occur with release of great amounts of energy; and (2) that in
the process extra neutrons were set free which might start a
chain reaction. It was not contrary to any known principle that
such a reaction should take place and that it should have very
important military application as a bomb. However, the idea
was revolutionary and therefore suspect; it was certain that
many technical operations of great difficulty would have to be
worked out before such a bomb could be produced. Probably
the only materials satisfactory for a bomb were either U-235,
which would have to be separated from the 140- times more
abundant isotope U-238, or Pu-239, an isotope of the hitherto
unknown element plutonium, which would have to be generated
by a controlled chain-reacting process itself hitherto unknown.
To achieve such a controlled chain reaction it was clear that
uranium metal and heavy water or beryllium or carbon might
have to be produced in great quantity with high purity. Once
bomb material was produced a process would have to be devel-
44 Statement of the Problem
oped for using it safely and effectively. In some of the processes,
health hazards of a new kind would be encountered.
POLICY PROBLEM
2.37. By the summer of 1940 the National Defense Research
Committee had been formed and was asking many of the scien-
tists in the country to work on various urgent military problems.
Scientific personnel was limited although this was not fully
realized at the time. It was, therefore, really difficult to decide
at what rate work should be carried forward on an atomic
bomb. The decision had to be reviewed at frequent intervals
during the subsequent four years. An account of how these policy
decisions were made is given in Chapters III and V.
CHAPTER III. ADMINISTRATIVE HISTORT
UP TO DECEMBER 1941
INTEREST IN MILITARY POSSIBILITIES
3.1. The announcement of the hypothesis of fission and its
experimental confirmation took place in January 1939, as has
already been recounted in Chapter I. There was immediate
interest in the possible military use of the large amounts of energy
released in fission. At that time American-born nuclear physicists
were so unaccustomed to the idea of using their science for mili-
tary purposes that they hardly realized what needed to be done.
Consequently the early efforts both at restricting publication
and at getting government support were stimulated largely by
a small group of foreign-born physicists centering on L. Szilard
and including E. Wigner, E. Teller, V. F. Weisskopf, and E.
Fermi.
RESTRICTION OF PUBLICATION-
S'. In the spring of 1939 the group mentioned above enlisted
Niels Bohr's cooperation in an attempt to stop publication of
further data by voluntary agreement. Leading American and
British physicists agreed, but F. Joliot, France's foremost nuclear
physicist, refused, apparently because of the publication of one
letter in the Physical Review sent in before all Americans had been
brought into the agreement. Consequently publication con-
tinued freely for about another year although a few papers were
withheld voluntarily by their authors.
3.3. At the April 1940 meeting of the Division of Physical
Sciences of the National Research Council, G. Breit proposed
formation of a censorship committee to control publication in
all American scientific journals. Although the reason for this
45
46 Administrative History to 1941
suggestion was primarily the desire to control publication of
papers on uranium fission, the "Reference Committee" as finally
set up a little later that spring (in the National Research Coun-
cil) was a general one, and was organized to control publication
policy in all fields of possible military interest. The chairman of
the committee was L. P. Eisenhart; other members were G. Breit,
W. M. Clark, H. Fletcher, E. B. Fred, G. B. Pegram, H. C. Urey,
L. H. Weed, and E. G. Wever. Various subcommittees were
appointed, the first one of which had to do with uranium fission.
G. Breit served as chairman of this subcommittee; its other
members were J. W. Beams, L. J. Briggs, G. B. Pegram, H. C.
Urey, and E. Wigner. In general, the procedure followed was to
have the editors of various journals send copies of papers in this
field, in cases where the advisability of publication was in doubt,
either directly to Breit or indirectly to him through Eisenhart.
Breit then usually circulated them to all members of the sub-
committee for consideration as to whether or not they should be
published^ and informed the editors as to the outcome. This
arrangement was very successful in preventing publication and
was still nominally in effect, in modified form, in June 1945.
Actually the absorption of most physicists in this country into
war work of one sort of another soon reduced the number of
papers referred to the committee practically to the vanishing
point. It is of interest to note that this whole arrangement was a
purely voluntary one; the scientists of the country are to be
congratulated on their complete cooperation. It is to be hoped
that it will be possible after the war to publish these papers at
least in part so that their authors may receive proper professional
credit for their contributions.
INITIAL APPROACHES TO THE GOVERNMENT
THE FIRST COMMITTEE
3.4. On the positive side government interest and support of
research in nuclear physics the history is a much more com-
plicated one. The first contact with the government was made by
Pegram of Columbia in March 1939. Pegram telephoned to the
Administrative History to 1941 47
Navy Department and arranged for a conference between
representatives of the Navy Department and Fermi. The only
outcome of this conference was that the Navy expressed interest
and asked to be kept informed. The next attempt to interest the
government was stimulated by Szilard and Wigner. In July
1939 they conferred with A. Einstein, and a little later Einstein,
Wigner, and Sziiard discussed the problem with Alexander
Sachs of New York. In the fall Sachs, supported by a letter from
Einstein, explained to President Roosevelt the desirability of
encouraging work in this field. The President appointed a com-
mittee, known as the "Advisory Committee on Uranium" and
consisting of Briggs (director of the Bureau of Standards) as chair-
man, Colonel K. F. Adamson of the Army Ordnance Department,
and Commander G. C. Hoover of the Navy Bureau of Ordnance,
and requested this committee to look into the problem. This was
the only committee on uranium that had official status up to the
time of organization of the National Defense Research Committee
in June 1940. The committee met very informally and included
various additional scientific representatives in its meetings.
3.5. The first meeting of the Uranium Committee was on
October 21, 1939 and included, besides the committee members,
F. L. Mohler, Alexander Sachs, L. Szilard, E. Wigner, E. Teller,
and R. B. Roberts. The result of this meeting was a report dated
November 1, 1939, and transmitted to President Roosevelt by
Briggs, Adamson, and Hoover. This report made eight recom-
mendations, which need not be enumerated in detail. It is
interesting, however, that it specifically mentions both atomic
power and an atomic bomb as possibilities. It specifically recom-
mended procurement of 4 tons of graphite and 50 tons of uranium
oxide for measurements of the absorption cross section of carbon.
Others of the recommendations either were of a general nature
or were never carried out. Apparently a memorandum prepared
by Szilard was more or less the basis of the discussion at this
meeting.
3.6. The first transfer of funds ($6,000) from the Army and
Navy to purchase materials in accordance with the recommenda-
48 Administrative History to 1941
tion of November 1 is reported in a memorandum from Briggs
to General E. M. Watson (President Roosevelt's aide) on
February 20, 1940. The next meeting of the "Advisory Com-
mittee on Uranium" was on April 28, 1940 and was attended by
Sachs, Wigner, Pegram, Fermi, Szilard, Briggs, Admiral H. G.
Bowen, Colonel Adamson, and Commander Hoover. By the
time of this meeting two important new factors had come into
the picture. First, it had been discovered that the uranium fission
caused by neutrons of thermal velocities occurred in the U-235
isotope only. Second, it had been reported that a large section
of the Kaiser Wilhelm Institute in Berlin had been set aside for
research on uranium. Although the general tenor of the discus-
sion at this meeting seems to have been that the work should be
pushed more vigorously, no definite recommendations were
made. It was pointed out that the critical measurements on
carbon already under way at Columbia should soon give a result,
and the implication was that definite recommendations should
wait for such a result.
3.7. Within the next few weeks a number of people concerned,
particularly Sachs, urged the importance of greater support and
of better organization. Their hand was strengthened by the
Columbia results (as reported, for example, in a letter from Sachs
to General Watson on May 15, 1940) showing that the carbon
absorption was appreciably lower than had been previously
thought and that the probability of carbon being satisfactory
as a moderator was therefore considerable. Sachs was also
active in looking into the question of ore supply. On June 1, 1940,
Sachs, Briggs, and Urey met with Admiral Bowen to discuss
approaching officials of the Union Miniere of the Belgian Congo.
Such an approach was made shortly afterwards by Sachs.
3.8. The general status of the problem was discussed by a
special advisory group called together by Briggs at the National
Bureau of Standards on June 15, 1940. This meeting was attended
by Briggs, Urey, M. A. Tuve, Wigner, Breit, Fermi, Szilard,
and Pegram. "After full discussion, the recommendation of the
group to the Uranium Committee was that funds should be
Administrative History to 1941 49
sought to support research on the uranium-carbon experiment
along two lines:
(A) further measurements of the nuclear constants involved
in the proposed type of reaction ;
(B) experiments with amounts of uranium and carbon equal
to about one fifth to one quarter of the amount that could
be estimated as the minimum in which a chain reaction
would sustain itself.
"It was estimated that about $40,000 would be necessary for
further measurements of the fundamental constants and that
approximately $100,000 worth of metallic uranium and pure
graphite would be needed for the intermediate experiment."
(Quotations from memorandum of Pegram to Briggs, dated
August 14, 1940.)
THE COMMITTEE RECONSTITUTED UNDER NDRC
3.9. Before any decisions made at this meeting could be put
into effect, the organization of the National Defense Research
Committee was announced in June 1 940, and President Roosevelt
gave instructions that the Uranium Committee should be recon-
stituted as a subcommittee of the NDRC, reporting to Vannevar
Bush (chairman, NDRC). The membership of this recon-
stituted Uranium Committee was as follows: Briggs, Chairman;
Pegram, Urey, Beams, Tuve, R. Gunn and Breit. On authoriza-
tion from Briggs, Breit consulted Wigner and Teller frequently
although they were not members of the committee. From that
time until the summer of 1941 this committee continued in
control with approximately the same membership. Its recom-
mendations were transmitted by Briggs to the NDRC, and suit-
able contracts were made between the NDRC and various
research institutions. The funds, however, were first supplied by
the Army and Navy, not from regular NDRC appropriations.
SUPPORT OF RESEARCH
3.10. The first contract let under this new set-up was to
Columbia University for the two lines of work recommended
50 Administrative History to 1941
at the June 15 meeting as described above. The project was
approved by the NDRC and the first NDRC contract (NDCrc-32)
was signed November 8, 1940, being effective from November 1,
1940, to November 1, 1941. The amount of this contract was
$40,000.
3.11. Only very small expenditures had been made before
the contract went into effect. For example, about $3,200 had
been spent on graphite and cadmium, this having been taken
from the $6,000 allotted by the Army and Navy in February,
1940.
3.12. We shall not attempt to review in detail the other con-
tracts that were arranged prior to December 1941. Their number
and total amount grew gradually. Urey began to work on isotope
separation by the centrifuge method under a Navy contract in
the fall of 1940. Other contracts were granted to Columbia
University, Princeton University, Standard Oil Development
Company, Cornell University, Carnegie Institution of Wash-
ington, University of Minnesota, Iowa State College, Johns
Hopkins University, National Bureau of Standards, University
of Virginia, University of Chicago, and University of California
in the course of the winter and spring of 1940-1941 until by
November 1941 the total number of projects approved was
sixteen, totalling about $300,000.
3.13. Scale of expenditure is at least a rough index of activity.
It is therefore interesting to compare this figure with those in
other branches of war research. By November 1941 the total
budget approved by NDRC for the Radiation Laboratory at
the Massachusetts Institute of Technology was several million
dollars. Even a relatively small project like that of Section S of
Division A of the NDRC had spent or been authorized to spend
$136,000 on work that proved valuable but was obviously not
potentially of comparable importance to the uranium work.
COMMITTEE REORGANIZED IN SUMMER OF 1941
3.14. The Uranium Committee as formed in the summer of
1940 continued substantially unchanged until the summer of
Administrative History to 1941 51
1941. At that time the main committee was somewhat enlarged
and subcommittees formed on isotope separation, theoretical
aspects, power production and heavy water.* It was thereafter
called the Uranium Section or the S-l Section of NDRC.
Though not formally disbanded until the summer of 1942, this
revised committee was largely superseded in December 1941
(see Chapter V).
THE NATIONAL ACADEMY REVIEWING COMMITTEE
3.15. In the spring of 1941, Briggs, feeling that an impartial
review of the problem was desirable, requested Bush to appoint
a reviewing committee. Bush then formally requested F. B.
Jewett, president of the National Academy of Sciences, to
appoint such a committee. Jewett complied, appointing A. H.
Compton, chairman; W. D. Coolidge, E. O. Lawrence, J. C.
Slater, J. H. Van Vleck, and B. Gherardi. (Because of illness,
Gherardi was unable to serve.) This committee was instructed to
evaluate the military importance of the uranium problem and
to recommend the level of expenditure at which the problem
should be investigated.
3.16. This committee met in May and submitted a report.
(This report and the subsequent ones will be summarized in the
next chapter.) On the basis of this report and the oral exposition
by Briggs before a meeting of the NDRC, an appropriation of
$267,000 was approved by the NDRC at its meeting of July 18,
1941, and the probability that much larger expenditures would
be necessary was indicated. Bush asked for a second report with
emphasis on engineering aspects, and in order to meet this
request O. E. Buckley of the Bell Telephone Laboratories and
* Uranium Section: Briggs, chairman; Pegram, vice-chairman; S. K.
Allison, Beams, Breit, E. U. Condon, H. D. Smyth, Urey.
Separation Subsection: Urey, chairman; Beams.
Power Production Subsection: Pegram, chairman; Allison, Fermi,
Smyth, Szilard.
Heavy Water Subsection: Urey, chairman; T. H. Chilton.
Theoretical Aspects Subsection: Fermi, chairman; Breit, C. H. Eckart,
Smyth, Szilard, J. A. Wheeler.
52 Administrative History to 1941
L. W. Chubb of the Westinghouse Electrical and Manufacturing
Company were added to the committee. (Compton was in South
America during the summer and therefore did not participate in
the summer meetings of the committee.) The second report was
submitted by Coolidge. As a result of new measurements of the
fission cross section of U-235 and of increasing conviction that
isotope separation was possible, in September 1941, Compton
and Lawrence suggested to J. B. Conant of NDRC, who was
working closely with Bush, that a third report was desirable. Since
Bush and Conant had learned during the summer of 1941 that
the British also felt increasingly optimistic, the committee was
asked to make another study of the whole subject. For this pur-
pose the committee was enlarged by the addition of W. K. Lewis,
R. S. Mulliken, and G. B. Kistiakowsky. This third report was
submitted by Compton on November 6, 1941.
INFORMATION RECEIVED FROM THE BRITISH
3.17. Beginning in 1940 there was some interchange of infor-
mation with the British and during the summer of 1941 Bush
learned that they had been reviewing the whole subject in the
period from April to July. They too had been interested in
the possibility of using plutonium; in fact, a suggestion as to the
advisability of investigating plutonium was contained in a letter
from J. D. Cockcroft to R. H. Fowler dated December 28, 1940.
Fowler, who was at that time acting as British scientific liaison
officer in Washington, passed Cockcroft's letter on to Lawrence.
The British never pursued the plutonium possibility, since they
felt their limited manpower should concentrate on U-235.
Chadwick, at least, was convinced that a U-235 bomb of great
destructive power could be made, and the whole British group
felt that the separation of U-235 by diffusion was probably feasible.
3.18. Accounts of British opinion, including the first draft of
the British report reviewing the subject, were made available to
Bush and Conant informally during the summer of 1941, although
the official British report of July 1 5 was first transmitted to Conant
by G. P. Thomson on October 3. Since, however, the British
Administrative History to 1941 53
review was not made available to the committee of the National
Academy of Sciences, the reports by the Academy committee
and the British reports constituted independent evaluations of the
prospects of producing atomic bombs.
3.19. Besides the official and semi-official conferences, there
were many less formal discussions held, one of these being stimu-
lated by M. L. E. Oliphant of England during his visit to this
country in the summer of 1 941 . As an example of such informal
discussion we might mention talks among Conant, Compton,
and Lawrence at the University of Chicago semicentennial cele-
bration in September 1941. The general conclusion was that the
program should be pushed; and this conclusion in various forms
was communicated to Bush by a number of persons.
3.20. In the fall of 1 941 Urey and Pegram were sent to England
to get first-hand information on what was being done there.
This was the first time that any Americans had been to England
specifically in connection with the uranium problem. The report
prepared by Urey and Pegram confirmed and extended the
information that had been received previously.
DECISION TO ENLARGE AND REORGANIZE
3.21. As a result of the reports prepared by the National
Academy committee, by the British, and by Urey and Pegram,
and of the general urging by a number of physicists, Bush, as
Director of the Office of Scientific Research and Development
(of which NDRG is a part), decided that the uranium work
should be pushed more aggressively.
3.22. Before the National Academy issued its third report and
before Pegram and Urey visited England, Bush had taken up
the whole uranium question with President Roosevelt and Vice-
President Wallace. He summarized for them the British views,
which were on the whole optimistic, and pointed out the un-
certainties of the predictions. The President agreed that it was
desirable to broaden the program, to provide a different organiza-
tion, to provide funds from a special source, and to effect com-
plete interchange of information with the British. It was agreed
54 Administrative History to 1941
to confine discussions of general policy to the following group:
The President, Vice-President, Secretary of War, Chief of Staff,
Bush, and Conant. This group was often referred to as the Top
Policy Group.
3.23. By the time of submission of the National Academy's
third report and the return of Urey and Pegram from England,
the general plan of the reorganization was beginning to emerge.
The Academy's report was more conservative than the British
report, as Bush pointed out in his letter of November 27, 1941, to
President Roosevelt. It was, however, sufficiently optimistic to
give additional support to the plan of enlarging the work. The
proposed reorganization was announced at a meeting of the
Uranium Section just before the Pearl Harbor attack and will be
described in Chapter V.
SUMMARY
3.24. In March 1939, only a few weeks after the discovery of
uranium fission, the possible military importance of fission was
called to the attention of the government. In the autumn of 1939
the first government committee on uranium was created. In the
spring of 1 940 a mechanism was set up for restricting publication
of significant articles in this field. When the NDRC was set up
in June 1940, the Uranium Committee was reconstituted under
the NDRC. However, up to the autumn of 1941 total expendi-
tures were relatively small. In December 1941, after receipt of
the National Academy report and information from the British,
the decision was made to enlarge and reorganize the program.
CHAPTER IV. PROGRESS UP TO
DECEMBER 1941
THE IMMEDIATE QUESTIONS
4.1. In Chapter II the general problems involved in producing
a chain reaction for military purposes were described. Early in
the summer of 1 940 the questions of most immediate importance
were:
(1) Could any circumstances be found under which the chain
reaction would go?
(2) Could the isotope U-235 be separated on a large scale?
(3) Could moderator and other materials be obtained in
sufficient purity and quantity?
Although there were many subsidiary problems, as will appear
in the account of the progress made in the succeeding eighteen
months, these three questions determined the course of the work.
THE CHAIN REACTION
PROGRAM PROPOSED JUNE 15, 1940
4.2. In June 1940, nearly all work on the chain reaction was
concentrated at Columbia under the general leadership of
Pegram, with Fermi and Szilard in immediate charge. It had
been concluded that the most easily produced chain reaction
was probably that depending on thermal neutron fission in a
heterogeneous mixture of graphite and uranium. In the spring
of 1940 Fermi, Szilard and H. L. Anderson had improved the
accuracy of measurements of the capture cross section of carbon
for neutrons, of the resonance (intermediate-speed) absorption
of neutrons by U-238, and of the slowing down of neutrons in
carbon.
55
56 Progress to December 1941
4.3. Pegram, in a memorandum to Briggs on August 14, 1940,
wrote, "It is not very easy to measure , these quantities with
accuracy without the use of large quantities of material. The
net results of these experiments in the spring of 1940 were that
the possibility of the chain reaction was not definitely proven,
while it was still further from being definitely disproven. On
the whole, the indications were more favorable than any con-
clusions that could fairly have been claimed from previous
results."
4.4. At a meeting on June 15 (see Chapter III) these results
were discussed and it was recommended that (A) further measure-
ments be made on nuclear constants, and (B) experiments be
made on lattices of uranium and carbon containing amounts of
uranium from one fifth to one quarter the estimated critical
amounts.
PROGRESS UP TO FEBRUARY 15, 1941
4.5. Pegram's report of February 15, 1941 shows that most
of the work done up to that time was on (A), while (B), the
so-called intermediate experiment, was delayed by lack of
materials.
4.6. Paraphrasing Pegram's report, the main progress was as
follows:
(a) The slowing down of neutrons in graphite was investigated by
studying the intensity of activation of various detectors (rhodium,
indium, iodine) placed at various positions inside a rectangular
graphite column of dimensions 3X3X8 feet when a source of
neutrons was placed therein. By suitable choice of cadmium
screens the effects of resonance* and thermal neutrons were
investigated separately, f A mathematical analysis, based on dif-
* See footnote p. 58.
f The presence of neutrons can be detected by ionization chambers or
counters or by the artificial radioactivity induced in various metal foils.
(See Appendix 1.) The response of each of these detectors depends on the
particular characteristics of the detector and on the speed of the neutrons
(e.g., neutrons of about 1.5 volts energy are particularly effective in acti-
vating indium). Furthermore, certain materials have very large absorption
(Progress to December 1941 57
fusion theory, of the experimental data made it possible to predict
the results to be expected in various other arrangements. These
results, coupled with theoretical studies of the diffusion of thermal
neutrons, laid a basis for future calculations of the number of
thermal and resonance neutrons to be found at any point in a
graphite mass of given shape when a given neutron source is
placed at a specified position within or near the graphite. ,
(b) The number of neutrons emitted in fission. The experiments on
slowing down neutrons showed that high-energy (high-speed)
neutrons such as those from fission were practically all reduced
to thermal energies (low speeds) after passing through 40 cm or
more of graphite. A piece of uranium placed in a region where
thermal neutrons are present absorbs the thermal neutrons and
as fission occurs re-emits fast neutrons, which are easily-
distinguished from the thermal neutrons. By a series of measure-
ments with and without uranium present and with various
detectors and absorbers, it is possible to get a value for the con-
stant 77, the number of neutrons emitted per thermal neutron
absorbed by uranium. This is not the number of neutrons emitted
per fission, but is somewhat smaller than that number since not
every absorption causes fission.
(c) Lattice theory. Extensive calculations were made on the
probable number of neutrons escaping from lattices of various
designs and sizes. This was fundamental for the so-called inter-
mediate experiment, mentioned above as item (B).
INITIATION OF NEW PROGRAMS
4.7. Early in 1941 interest in the general chain-reaction
problem by individuals at Princeton, Chicago, and California
led to the approval of certain projects at those institutions.
Thereafter the work of these groups was coordinated with the
work at Columbia, forming parts of a single large program.
cross sections for neutrons of particular ranges of speed (e.g., cadmium for
thermal neutrons). Thus measurements with different detectors with or
without various absorbers give some indication of both the number of
neutrons present and their energy distribution. However, the state of the art
of such measurements is rather crude.
58 Progress to December 1941
WORK ON RESONANCE ABSORPTION*
4.8. In Chapter II it is stated that there were advantages in a
lattice structure or "pile" with uranium concentrated in lumps
regularly distributed in a matrix of moderator. This was the
system on which the Columbia group was working. As is so often
the case, the fundamental idea is a simple one. If the uranium
and the moderator are mixed homogeneously, the neutrons on
the average will lose energy in small steps between passages
through the uranium so that in the course of their reduction to
thermal velocity the chance of their passing through uranium at
any given velocity, e.g., at a velocity corresponding to resonance
absorption, is great. But, if the uranium is in large lumps spaced
at large intervals in the moderator, the amounts of energy lost by
neutrons between passages from one lump of uranium to another
will be large and the chance of their reaching a uranium lump
with energy just equal to the energy of resonance absorption is
relatively small. Thus the chance of absorption by U-238 to
produce U-239, compared to the chance of absorption as thermal
neutrons to cause fission, may be reduced sufficiently to allow a
chain reaction to take place. If one knew the exact values of the
cross sections of each uranium isotope for each type of absorption
and every range of neutron speed, and had similar knowledge for
the moderator, one could calculate the "optimum lattice," i.e.,
the best size, shape and spacing for the lumps of uranium in the
matrix of moderator. Since such data were only partially known,
a direct experimental approach appeared to be in order. Conse-
quently it was proposed that the absorption of neutrons by
uranium should be measured under conditions similar to those ex-
pected in a chain-reacting pile employing graphite as moderator.
4.9. Experiments of this type were initiated at Columbia, and
were continued at Princeton in February 1941. Essentially the
* The term "resonance absorption" is used to describe the very strong
absorption of neutrons by U-238 when the neutron energies are in certain
definite portions of the energy region from to 1,000 electron volts. Such
resonance absorption demonstrates the existence of nuclear energy levels at
corresponding energies. On some occasions the term resonance absorption
is used to refer to the whole energy region in the neighborhood of such levels.
Progress to December 1941 59
experiment consisted of studying the absorption of neutrons in
the energy range extending from a few thousand electron volts
down to a fraction of an electron volt (thermal energies), the
absorption taking place in different layers of uranium or uranium
oxide spheres embedded in a pile of graphite.
4.10. In these experiments, a source of neutrons was provided
by a mean of protons (accelerated by a cyclotron) impinging on
a beryllium target. (The resulting yield of neutrons was equiva-
lent to the yield from a radium-beryllium source of about 3,500
curies strength.) The neutrons thus produced had a wide, con-
tinuous, velocity distribution. They proceeded from this source
into a large block of graphite. By placing the various uranium or
uranium-oxide spheres inside the graphite block at various
positions representing increasing distances from the source,
absorption of neutrons of decreasing average speeds down to
thermal speeds was studied. It was found that the total absorption
of neutrons by such spheres could be expressed in terms of a
"surface" effect and a "mass" effect.
4.11. These experiments, involving a variety of sphere sizes,
densities, and positions were continued until the spring of 1942,
when most of the group was moved to Chicago. Similar experi-
ments performed at a later date at the University of Indiana by
A. C. G. Mitchell and his co-workers have verified and in some
cases corrected the Princeton data, but the Princeton data were
sufficiently accurate by the summer of 1941 to be used in plan-
ning the intermediate-pile experiments and the subsequent ex-
periments on operating piles.
4.12. The experimental work on resonance absorption at
Princeton was done by R. R. Wilson, E. C. Creutz, and their
collaborators, under the general leadership of H. D. Smyth; they
benefited from the constant help of Wigner and Wheeler and
frequent conferences with the Columbia group.
THE FIRST INTERMEDIATE EXPERIMENTS
4.13. About July 1941 the first lattice structure of graphite and
uranium was set up at Columbia. It was a graphite cube about
60 Progress to December 1941
8 feet on an edge, and contained about 7 tons of uranium oxide
in iron containers distributed at equal intervals throughout the
graphite. A preliminary set of measurements was made on this
structure in August 1941. Similar structures of somewhat larger
size were set up and investigated during September and October,
and the so-called exponential method (described below) of
determining the multiplication factor was developed and first
applied. This work was done by Fermi and his assistants, H. L.
Anderson, B. Feld, G. Weil, and W. H. Zinn.
4.14. The multiplication-factor experiment is rather similar
to that already outlined for the determination of 17, the number
of neutrons produced per thermal neutron absorbed. A radium-
beryllium neutron source is placed near the bottom of the lattice
structure and the number of neutrons is measured at various
points throughout the lattice. These numbers are then compared
with the corresponding numbers determined when no uranium
is present in the graphite mass. Evidently the absorption of
neutrons by U-238 to produce U-239 tends to reduce the number
of neutrons, while the fissions tend to increase the number.
The question is: Which predominates? or, more precisely, Does
the fission production of neutrons predominate over all neutron-
removal processes other than escape? Interpretation of the experi-
mental data on this crucial question involves many corrections,
calculations, and approximations, but all reduce in the end to a
single number, the multiplication factor k.
THE MULTIPLICATION FACTOR K
4.15. The whole success or failure of the uranium project
depended on the multiplication factor k, sometimes called the
reproduction factor. If k could be made greater than 1 in a
practical system, the project would succeed; if not, the chain
reaction would never be more than a dream. This is clear from
the following discussion, which applies to any system containing
fissionable material. Suppose that there is a certain number of
free neutrons present in the system at a given time. Some of
these neutrons will themselves initiate fissions and will thus
Progress to December 1941 61
directly produce new neutrons. The multiplication factor k is the
ratio of the number of these new neutrons to the number of free
neutrons originally present. Thus, if in a given pile comprising
uranium, carbon, impurities, containers, etc., 100 neutrons are
produced by fission, some will escape, some will be absorbed in
the uranium without causing fission, some will be absorbed in
the carbon, in the containers or in impurities, and some will
cause fission, thereby producing more neutrons.* If the fissions
are sufficiently numerous and sufficiently effective individually,
more than 100 new neutrons will be produced and the system is
chain reacting. If the number of new neutrons is 105, k = 1.05.
But if the number of new neutrons per 100 initial ones is 99,
k = .99 and no chain reaction can maintain itself.
4.16. Recognizing that the intermediate or "exponential"
experiment described above was too small to be chain reacting,
we see that it was a matter of great interest whether any larger
pile of the same lattice structure would be chain reacting. This
could be determined by calculating what the value of k would be
for an infinitely large lattice of this same type. In other words,
the problem was to calculate what the value of k would be if no
neutrons leaked away through the sides of the pile. Actually it is
found that, once a chain-reacting system is well above the critical
size say two or three times as great and is surrounded by what
is called a reflector, the effective value of k differs very little from
that for infinite size provided that k is near 1.00. Consequently,
it has become customary to characterize the chain-reaction
potentialities of different mixtures of metal and moderator by
the value of k* the multiplication constant obtained by assuming
infinite size of pile.
4.17. The value of k* as reported by Fermi to the Uranium
Section in the fall of 1941 was about 0.87. This was based on
results from the second Columbia intermediate experiment. All
agreed that the multiplication factor could be increased by greater
purity of materials, different lattice arrangements, etc. None
could say with certainty that it could be made greater than one.
* See drawing facing p. 35.
62 Progress to December 1941
EXPERIMENTS ON BERYLLIUM
4.18. At about the same time that the work on resonance
absorption was started at Princeton, S. K. Allison, at the sugges-
tion of A. H. Compton, began work at Chicago under a contract
running from January 1, 1941 to August 1, 1941. The stated
objectives of the work were to investigate (a) the increase in
neutron production when the pile is enclosed in a beryllium
envelope or "reflector," and (b) the cross sections of beryllium.
A new contract was authorized on July 18, 1941, to run to June
30, 1942. This stated the somewhat broader objective of investi-
gating uranium-beryllium-carbon systems generally. The appro-
priations involved were modest: $9,500 for the first contract, and
$30,000 for the second contract.
4.19. As has already been pointed out in Chapter II, beryllium
has desirable qualities as a moderator because of its low atomic
weight and low neutron-absorption cross section; there was also
the possibility that a contribution to the number of neutrons
would be realized from the (n, 2n) reaction in beryllium. The
value of the cross section was not precisely known; furthermore
it was far from certain that any large amount of pure beryllium
could be obtained. Allison's problem was essentially similar to the
Columbia problem, except for the use of beryllium in place of
graphite. Because of the scarcity of beryllium it was suggested
that it might be used in conjunction with graphite or some other
moderator, possibly as a reflector.
4.20. In the Chicago experiments, neutrons produced with the
aid of a cyclotron were caused to enter a pile of graphite and
beryllium. Allison made a number of measurements on the
slowing down and absorption by graphite which were valuable
checks on similar experiments at Columbia. He finally was able
to obtain enough beryllium to make significant measurements
which showed that beryllium was a possible moderator com-
parable to graphite. However, beryllium was not in fact used at
all extensively in view of the great difficulty of producing it in
quantity in the required structural forms.
Progress to December 1941 63
4.21. This Chicago project as described above became part
of the Metallurgical Laboratory project established at the Uni-
versity of Chicago early in 1942.
THEORETICAL WORK
4.22. Both the intermediate experiments at Columbia and the
continued resonance-absorption work at Princeton required
skilful theoretical interpretation. Fermi worked out the theory
of the "exponential" pile and Wigner the theory of resonance
absorption; both these men were constantly conferring and
contributing to many problems. Wheeler of Princeton, Breit of
Wisconsin, and Eckart of Chicago to mention only a few also
made contributions to general pile theory and related topics.
Altogether one can say that by the end of 1941 the general theory
of the chain reaction for slow neutrons was almost completely
understood. It was the numerical constants and technological
possibilities that were still uncertain.
4.23. On the theory of a fast-neutron reaction in U-235 a good
deal of progress had also been made. In particular, new estimates
of the critical size were made, and it was predicted that possibly
10 per cent of the total energy might be released explosively.
On this basis one kilogram of U-235 would be equivalent to
2,000 tons of TNT. The conclusions are reviewed below in con-
nection with the National Academy Report. It is to be remem-
bered that there are two factors involved: (1) how large a fraction
of the available fission energy will be released before the reaction
stops; (2) how destructive such a highly concentrated explosion
will be.
WORK ON PLUTONIUM
4.24. In Chapter I mention is made of the suggestion that the
element 94, later christened plutonium, would be formed by
beta-ray disintegrations of U-239 resulting from neutron absorp-
tion by U-238 and that plutonium would probably be an alpha-
particle emitter of long half-life and would undergo fission when
bombarded by neutrons. In the summer of 1940 the nuclear
64 Progress to December 1941
physics group at the University of California in Berkeley was
urged to use neutrons from its powerful cyclotron for the pro-
duction of plutonium, and to separate it from uranium and
investigate its fission properties. Various pertinent experiments
were performed by E. Se'gre, G. T. Seaborg, J. W. Kennedy,
and A. C. Wahl at Berkeley prior to 1941 and were reported by
E. O. Lawrence to the National Academy Committee (see below)
in May 1941 and also in a memorandum that was incorporated
in the Committee's second report dated July 11, 1941. It will be
seen that this memorandum includes one important idea not
specifically emphasized by others (paragraph 1.58), namely,
the production of large quantities of plutonium for use in a bomb.
4.25. We quote from Lawrence's memorandum as follows:
"Since the first report of the National Academy of Sciences Com-
mittee on Atomic Fission, an extremely important new possibility
has been opened for the exploitation of the chain reaction with
unseparated isotopes of uranium. Experiments in the Radiation
Laboratory of the University of California have indicated (a) that
element 94 is formed as a result of capture of a neutron by
uranium 238 followed by two successive beta-transformations,
and furthermore (b) that this transuranic element undergoes
slow neutron fission and therefore presumably behaves like
uranium 235.
"It appears accordingly that, if a chain reaction with unsepa-
rated isotopes is achieved, it may be allowed to proceed violently
for a period of time for the express purpose of manufacturing
element 94 in substantial amounts. This material could be ex-
tracted by ordinary chemistry and would presumably be the
equivalent of uranium 235 for chain reaction purposes.
"If this is so, the following three outstanding important possi-
bilities are opened:
"1. Uranium 238 would be available for energy production,
thus increasing about one hundred fold the total atomic energy
obtainable from a given quantity of uranium.
"2. Using element 94 one may envisage preparation of small
chain reaction units for power purposes weighing perhaps a
Progress to December 1941 65
hundred pounds instead of a hundred tons as probably would be
necessary for units using natural uranium.
"3. If large amounts of element 94 were available it is likely
that a chain reaction with fast neutrons could be produced. In
such a reaction the energy would be released at an explosive
rate which might be described as 'super bomb.' "
RADIOACTIVE POISONS
4.26. As previously stated, the fragments resulting from fission
are in most cases unstable nuclei, that is, artificially radioactive
materials. It is common knowledge that the radiations from
radioactive materials have deadly effects akin to the effects of
X-rays.
4.27. In a chain-reacting pile these radioactive fission products
build up as the reaction proceeds. (They have, in practice, turned
out to be the most troublesome feature of a reacting pile.) Since
they differ chemically from the uranium, it should be possible
to extract them and use them like a particularly vicious form of
poison gas. This idea was mentioned in the National Academy
report (see paragraph 4.48) and was developed in a report
written December 10, 1941, by E. Wigner and H. D. Smyth,
who concluded that the fission products produced in one day's
run of a 100,000 kw chain-reacting pile might be sufficient to
make a large area uninhabitable.
4.28. Wigner and Smyth did not recommend the use of radio-
active poisons nor has such use been seriously proposed since
by the responsible authorities, but serious consideration was
given to the possibility that the Germans might make surprise
use of radioactive poisons, and accordingly defensive measures
were planned.
ISOTOPE SEPARATION
SMALL-SCALE SEPARATION BY THE MASS SPECTROGRAPH
4.29. In Chapter I the attribution of thermal-neutron fission
of uranium to the U-235 isotope was mentioned as being experi-
66 Progress to December 1941
mentally established. This was done by partly separating minute
quantities of the uranium isotopes in A. O. Nier's mass specto-
graph and then studying the nuclear properties of the samples.
Additional small samples were furnished by Nier in the summer
of 1941 and studied by N. P. Heydenburg and others at M. A.
Tuve's laboratory at the Department of Terrestrial Magnetism
of the Carnegie Institution of Washington. But results of such
experiments were still preliminary, and it was evident that
further study of larger and more completely separated samples
was desirable.
4.30. The need of larger samples of U-235 stimulated E. O.
Lawrence at Berkeley to work on electromagnetic separation.
He was remarkably successful and by December 6, 1941 reported
that he could deposit in one hour one microgram of U-235 from
which a large proportion of the U-238 had been removed.
4.31. Previously, at a meeting of the Uranium Committee,
Smyth of Princeton had raised the question of possible large-scale
separation of isotopes by electromagnetic means but had been
told that it had been investigated and was considered impossible.
Nevertheless, Smyth and Lawrence at a chance meeting in
October 1941 discussed the problem and agreed that it might yet
be possible. Smyth again raised the question at a meeting of the
Uranium Committee on December 6 and at the next meeting
(December 18, 1941) there was a general discussion of large-scale
electromagnetic methods in connection with Lawrence's report
of his results already mentioned. The consequences of this dis-
cussion are reported in Chapter XI.
THE CENTRIFUGE AND GASEOUS DIFFUSION METHODS
4.32. Though we have made it clear that the separation of
U-235 from U-238 might be fundamental to the whole success of
the project, little has been said about work in this field. Such work
had been going on since the summer of 1940 under the general
direction of H. C. Urey at Columbia. Since this part of the ura-
nium work was not very much affected by the reorganization in
Progress to December 1941 67
December 1941, a detailed account of the work is reserved for
Chapters IX and X. Only a summary is presented here.
4.33. After careful review and a considerable amount of
experimenting on other methods, it had been concluded that the
two most promising methods of separating large quantities of
U-235 from U-238 were by the use of centrifuges and by the use
of diffusion through porous barriers. In the centrifuge, the forces
acting on the two isotopes are slightly different because of their
differences in mass. In the diffusion through barriers, the rates of
diffusion are slightly different for the two isotopes, again because
of their differences in mass. Each method required the uranium to
be in gaseous form, which was an immediate and serious limita-
tion since the only suitable gaseous compound of uranium then
known was uranium hexafluoride. In each method the amount
of enrichment to be expected in a single production unit or "stage"
was very small; this indicated that many successive stages would
be necessary if a high degree of enrichment was to be attained.
4.34. By the end of 1941 each method had been experimentally
demonstrated in principle; that is, single-stage separators had
effected the enrichment of the U-235 on a laboratory scale to
about the degree predicted theoretically. K. Cohen of Columbia
and others had developed the theory for the single units and for
the series or "cascade" of units that would be needed. Thus it
was possible to estimate that about 5,000 stages would be neces-
sary for one type of diffusion system and that a total area of
many acres of diffusion barrier would be required in a plant
separating a kilogram of U-235 each day. Corresponding cost
estimates were tens of millions of dollars. For the centrifuge the
number of stages would be smaller, but it was predicted that a
similar production by centrifuges would require 22,000 separately
driven, extremely high-speed centrifuges, each three feet in length
at a comparable cost.
4.35. Of course, the cost estimates could not be made accu-
rately since the technological problems were almost completely
unsolved, but these estimates as to size and cost of plant did serve
to emphasize the magnitude of the undertaking.
68 Progress to December 1941
THERMAL DIFFUSION IN LIQUIDS
4.36. In September 1940, P. H. Abelson submitted to Briggs a
17-page memorandum suggesting the possibility of separating
the isotopes of uranium by thermal diffusion in liquid uranium
hexafluoride. R. Gunn of the Naval Research Laboratory was
also much interested in the uranium problem and was appointed
a member of the Uranium Committee when it was reorganized
under the NDRC in the summer of 1940. As a result of Abelson's
suggestion and Gunn's interest, work was started on thermal dif-
fusion at the National Bureau of Standards. This work was
financed by funds from the Navy Department and in 1940 was
transferred to the Naval Research Laboratory, still under the
direction of Abelson, where it was continued.
4.37. We shall discuss the thermal-diffusion work further in a
later chapter, but we may mention here that significant results
had already been obtained by the end of 1941 and that in Janu-
ary 1942, using a single separation column, a separation factor
had been obtained which was comparable or superior to the one
obtained up to that time in preliminary tests on the diffusion
and centrifuge methods.
THE PRODUCTION OF HEAVY WATER
4.38. It was pointed out in Chapter II that deuterium ap-
peared very promising as a moderator because of its low absorp-
tion and good slowing-down property but unpromising because
of its scarcity. Interest in a deuterium moderator was stimulated
by experimental results obtained in Berkeley demonstrating that
the deuterium absorption cross section for neutrons was, in fact,
almost zero. Since oxygen has a very low absorption coefficient
for neutrons, it was usually assumed that the deuterium would be
used combined with oxygen, that is, in the very convenient
material: heavy water. Work at Columbia on possible methods
of large-scale concentration of heavy water was initiated in
February 1941 under the direction of H. C. Urey (under an
OSRD contract). Early in 1941, R. H. Fowler of England re-
Progress to December 1941 69
ported the interest of the British group in a moderator of deu-
terium in the form of heavy water and their conviction that a
chain reaction would go in relatively small units of uranium and
heavy water.
4.39. Urey and A. von Grosse had already been considering
the concentration of heavy water by means of a catalytic ex-
change reaction between hydrogen gas and liquid water. This
process depends on the fact that, when isotopic equilibrium is
established between hydrogen gas and water, the water contains
from three to four times as great a concentration of deuterium as
does the hydrogen gas. During 1941, this exchange reaction
between water and hydrogen was investigated at Columbia and
in the Frick Chemical Laboratory at Princeton and extensive
work was done toward developing large-scale methods of pro-
ducing materials suitable for catalyzing the reaction.
4.40. The further development of this work and of other
methods of producing heavy water are discussed in Chapter IX.
Like the other isotope-separation work at Columbia, this work
was relatively unaffected by the reorganization in December
1941. It is mentioned in preliminary fashion here to indicate
that all the principal lines of approach were under investigation
in 1941.
PRODUCTION AND ANALYSIS OF MATERIALS
4.41. By the end of 1941 not very much progress had been
made in the production of materials for use in a chair-reacting
system. The National Bureau of Standards and the Columbia
group were in contact with the Metal Hydrides Company of
Beverly, Massachusetts. This company was producing some
uranium in powdered form, but efforts to increase its production
and to melt the powdered metal into solid ingots had not been
very successful.
4.42. Similarly, no satisfactory arrangement had been made
for obtaining large amounts of highly purified graphite. The
graphite in use at Columbia had been obtained from the U. S.
Graphite Company of Saginaw, Michigan. It was of high purity
70 Progress to December 1941
for a commercial product, but it did contain about one part in
500,000 of boron, which was undesirable.
4.43. Largely through the interest of Allison the possibility of
increasing the production of beryllium had been investigated to
the extent of ascertaining that it would be difficult and expensive,
but probably possible.
4.44. Though little progress had been made on procurement,
much progress had been made on analysis. The development of
sufficiently accurate methods of chemical analysis of the materials
used has been a problem of the first magnitude throughout the
history of the project, although sometimes overshadowed by the
more spectacular problems encountered. During this period
C. J. Rodden and others at the National Bureau of Standards
were principally responsible for analyses; H. T. Beans of Columbia
also cooperated. By 1942 several other groups had started analyt-
ical sections which have been continuously active ever since.
4.45. To summarize, by the end of 1941 there was no evidence
that procurement of materials in sufficient quantity and purity
was impossible, but the problems were far from solved.
EXCHANGE OF INFORMATION
WITH THE BRITISH
4.46. Prior to the autumn of 1941 there had been some ex-
change of reports with the British and some discussion with
British scientific representatives who were here on other business.
In September 1941, it was decided that Pegram and Urey should
get first-hand information by a trip to England. They completed
their trip in the first week of December 1941.
4.47. In general, work in England had been following much
the same lines as in this country. As to the chain-reaction problem,
their attention had focussed on heavy water as a moderator
rather than graphite; as to isotope separation, they had done ex-
tensive work on the diffusion process including the general theory
of cascades. Actually the principal importance of this visit and
other interchanges during the summer of 1941 lay not in accurate
Progress to December 1941 71
scientific data but in the general scientific impressions. The
British, particularly J. Chadwick, were convinced that a U-235
chain reaction could be achieved. They knew that several kilo-
grams of heavy water a day were being produced in Norway,
and that Germany had ordered considerable quantities of par-
affin to be made using heavy hydrogen; it was difficult to imagine
a use for these materials other than in work on the uranium
problem. They feared that if the Germans got atomic bombs
before the Allies did, the war might be over in a few weeks. The
sense of urgency which Pegram and Urey brought back with them
was of great importance.
THE NATIONAL ACADEMY
COMMITTEE REPORT
4.48. The appointment of a National Academy committee was
mentioned in Chapter III. The committee's first report in May
1941 mentioned (a) radioactive poisons, (b) atomic power, and
(c) atomic bombs, but the emphasis was on power. The second
report stressed the importance of the new results on plutonium,
but was not specific about the military uses to which the fission
process might be put. Both these reports urged that the project
be pushed more vigorously.
4.49. The third report (November 6, 1941) was specifically
concerned with the "possibilities of an explosive fission reaction
with U-235." Although neither of the first two National Academy
reports indicated that uranium would be likely to be of decisive
importance in the present war, this possibility was emphasized
in the third report. We can do no better than quote portions of
this report.
"Since our last report, the progress toward separation of the
isotopes of uranium has been such as to make urgent a considera-
tion of (1) the probability of success in the attempt to produce a
fission bomb, (2) the destructive effect to be expected from such
a bomb, (3) the anticipated time before its development can be
completed and production be underway, and (4) a preliminary
estimate of the costs involved."
72 Progress to December 1941
"1. Conditions for a fission bomb. A fission bomb of superlatively
destructive power will result from bringing quickly together a sufficient
mass of element U-235. This seems to be as sure as any untried
prediction based upon theory and experiment can be. Our
calculations indicate further that the required masses can be
brought together quickly enough for the reaction to become
efficient . . .
"2. Destructive effect of fission bombs, (a) Mass of the bomb. The
mass of U-235 required to produce explosive fission under appropriate con-
ditions can hardly be less than 2 kg nor greater than 100 kg. These wide
limits reflect chiefly the experimental uncertainty in the capture
cross section of U-235 for fast neutrons . . . (b) Energy released
by explosive fission. Calculations for the case of masses properly-
located at the initial instant indicate that between 1 and 5 per
cent of the fission energy of the uranium should be released at a
fission explosion. This means from 2 to 10 X 10 8 kilocalories per
kg of uranium 235. The available explosive energy per kg of uranium
is thus equivalent to about 300 tons of TNT.
"3. Time required for development and production of the necessary
U-235. (a) Amount of uranium needed. Since the destructiveness of
present bombs is already an important factor in warfare, it is
evident that, if the destructiveness of the bombs is thus increased
10,000-fold, they should become of decisive importance.
"The amount of uranium required will, nevertheless, be large.
If the estimate is correct that 500,000 tons of TNT bombs would
be required to devastate Germany's military and industrial
objectives, from 1 to 10 tons of U-235 will be required to do the same job.
"(b) Separation of U-235. The separation of the isotopes of uranium
can be done in the necessary amounts. Several methods are under
development, at least two of which seem definitely adequate,
and are approaching the stage of practical test. These are the
methods of the centrifuge and of diffusion through porous bar-
riers. Other methods are being investigated or need study which
may ultimately prove superior, but are now farther from the
engineering stage.
"(c) Time required for production of fission bombs. An estimate of
Progress to December 1941 73
time required for development, engineering and production of
fission bombs can be made only very roughly at this time.
"If all possible effort is spent on the program, one might
however expect fission bombs to be available in significant
quantity within three or four years.
"4. Rough estimate of costs. (The figures given in the Academy
report under this heading were recognized as only rough esti-
mates since the scientific and engineering data to make them
more precise were not available. They showed only that the
undertaking would be enormously expensive but still in line with
other war expenditures.)"
4.50. The report then goes on to consider immediate require-
ments and desirable reorganization.
SUMMARY
4.51. At the end of Chapter I we summarized the knowledge
of nuclear fission as of June 1940, and in Chapter II we stated
the outstanding problems as of the same date. In the light of
these statements we wish to review the eighteen months' progress
that has just been recounted. The tangible progress was not great.
No chain reaction had been achieved; no appreciable amount
of U-235 had been separated from U-238; only minute amounts of
Pu-239 had been produced; the production of large quantities of
uranium metal, heavy water, beryllium, and pure graphite was
still largely in the discussion stage. But there had been progress.
Constants were better known; calculations had been checked
and extended; guesses as to the existence and nuclear properties
of Pu-239 had been verified. Some study had been made of engi-
neering problems, process effectiveness, costs, and time schedules.
Most important of all, the critical size of the bomb had been
shown to be almost certainly within practical limits. Altogether
the likelihood that the problems might be solved seemed greater
in every case than it had in 1 940. Perhaps more important than
the actual change was the psychological change. Possibly Wigner,
Szilard, and Fermi were no more thoroughly convinced that
atomic bombs were possible than the> r had been in 1940, but
74 Progress to December 1941
many other people had become familiar with the idea and its
possible consequences. Apparently, the British and the Germans,
both grimly at war, thought the problem worth undertaking.
Furthermore, the whole national psychology had changed.
Although the attack at Pearl Harbor was yet to come, the im-
pending threat of war was much more keenly felt than before,
and expenditures of effort and money that would have seemed
enormous in 1940 were considered obviously necessary precau-
tions in December 1941. Thus it was not surprising that Bush and
his associates felt it was time to push the uranium project vigor-
ously. For this purpose, there was created an entirely new
administrative organization which will be described in the next
chapter.
CHAPTER V. ADMINISTRATIVE
HISTORY 1942-1945
5.1. In Chapter III the administrative history of the uranium
work up to December 1 941 was reviewed. Chapter IV reported
the progress of the scientific work up to the same date. The
present chapter describes the administrative reorganization that
took place in December 1941 and various changes that occurred
after that time.
REORGANIZATION OF NDRC URANIUM SECTION
TRANSFER TO OSRD
5.2. Two major decisions were required in the further plan-
ning of the uranium or atomic-bomb program. These decisions
were made by Vannevar Bush, Director of the Office of Scientific
Research and Development (which included NDRC), after
conference with various scientists and administrators concerned.
(See Chapter III.) The decisions were: first, that the possibility
of obtaining atomic bombs for use in the present war was great
enough to justify an "all out" effort for their development; second,
that the existing organization, the NDRC Uranium Section
(known as the S-l Section, and consisting of L. J. Briggs, chair-
man; G. B. Pegram, vice-chairman; H. T. Wensel, technical
aide; S. K. Allison, J. W. Beams, G. Breit, E. U. Condon, R.
Gunn, H. D. Smyth, and H. C. Urey) was not properly organized
for such an effort.
5.3. At a meeting of the National Defense Research Committee
on November 28, 1941, Dr. Bush explained why he felt that it
was desirable to set up the uranium program outside NDRC.
The members of NDRC agreed to a transfer. Accordingly, the
NDRC as an organization had no further connection with the
75
76 Administrative History 1942-1945
uranium program, which was administered for some time
thereafter by the OSRD directly through an OSRD S-l Section,
and later through an OSRD S-l Executive Committee.
5.4. At a meeting of the S-l Section of OSRD on December 6,
1941, J. B. Conant, speaking for Bush, announced the proposed
"all out" effort and the reorganization of the group. The S-l
Section itself had not been formally consulted on the proposed
reorganization, but there is no doubt that most of its members
were strongly in favor of the new proposals. The membership of
the reorganized S-l Section was as follows: J. B. Conant, repre-
sentative of V. Bush; L. J. Briggs, chairman; G. B. Pegram, vice-
chairman; A. H. Compton, program chief; E. O. Lawrence,
program chief; H. C. Urey, program chief; E. V. Murphree,
chairman of the separately organized Planning Board; H. T.
Wensel, technical aid; S. K. Allison, J. W. Beams, G. Breit,
E. U. Condon, H. D. Smyth.
FORMATION OF THE PLANNING BOARD
5.5. At the time the S-l Section was reorganized, Bush also
set up a Planning Board to be responsible for the technical and
engineering aspects of the work, for procurement of materials
and for construction of pilot plants and full-size production
plants. This Planning Board consisted of E. V. Murphree (chair-
man), W. K. Lewis, L. W. Chubb, G. O. Curme, Jr., and P. C.
Keith.
FUNCTIONS OF THE PLANNING BOARD
AND OSRD S-l SECTION
5.6. It was arranged that contracts for the scientific parts of
the work would be recommended to Bush not by the full S-l
Section but by Briggs and Conant after conferences with the
program chiefs involved and that recommendations on engineer-
ing contracts would be made to Bush by the Planning Board.
(The contracts which had been made on behalf of the old
Uranium Section had been administered through the NDRC.)
Administrative History 1942-1945 77
Contracts for the development of diffusion and centrifuge sepa-
ration processes were to be recommended by the Planning Board,
which would be responsible for the heavy-water production
program also. Bush stated that the Planning Board "will be
responsible for seeing to it that we have plans on which to proceed
with the next step as expeditiously as possible."
5.7. The scientific aspects of the work were separated from the
procurement and engineering phases. The Program Chiefs
H. C. Urey, E. O. Lawrence, and A. H. Compton were to have
charge of the scientific aspects. Initially it was proposed that Urey
should have charge of the separation of isotopes by the diffusion
and the centrifuge methods and of the research work on the
production of heavy water. Lawrence was to have charge of the
initial production of small samples of fissionable elements, of
quantity production by electromagnetic-separation methods, and
of certain experimental work relating to the properties of the
plutonium nucleus. Compton was to have charge of fundamental
physical studies of the chain reaction and the measurement of
nuclear properties with especial reference to the explosive chain
reaction. As an afterthought, he was authorized to explore also
the possibility that plutonium might be produced in useful
amounts by the controlled chain-reaction method. It was under-
stood, however, that this division of responsibility was to be more
precisely defined in later conferences. (The written records of
that period do not always give adequate accounts of what was in
the minds of the men concerned. In deference to security re-
quirements, references to the importance of plutonium and even
to the bomb itself were often omitted entirely.)
5.8. The effect of the reorganization was to put the direction
of the projects in the hands of a small group consisting of Bush,
Conant, Briggs, Compton, Urey, Lawrence, and Murphree.
Theoretically, Compton, Lawrence, Urey, and Murphree were
responsible only for their respective divisions of the program.
Each met with Conant and Briggs or occasionally with
Bush to discuss his specific problems, or even the overall
program.
78 Administrative History 1942-1945
MEETING OF TOP POLICY GROUP APPROVAL
OF REORGANIZATION
5.9. A meeting of the Top Policy Group, consisting of Vice-
President Henry A. Wallace, Secretary of War Henry L. Stimson,
and Dr. V. Bush, was held on December 16, 1941 . General George
C. Marshall and Dr. J. B. Conant, also members of the group,
were absent; Mr. H. L. Smith of the Budget Bureau attended.
Bush described the reorganization that was in progress and his
plans were approved. In a memorandum to Conant describing
this meeting, Bush wrote, "It was definitely felt by the entire
group that OSRD should press as fast as possible on the funda-
mental physics and on the engineering planning, and particularly
on the construction of pilot plants." Bush estimated the cost of
this aspect of the work would be four or five million dollars, and
stated the Army should take over when full-scale construction
was started, presumably when pilot plants were ready. He sug-
gested the assignment of a technically trained Army officer to
become familiar with the general nature of the uranium problem.
It was made clear at this meeting that the international relations
involved were in the hands of the President, with Bush responsible
for liaison on technical matters only.
MEETING OF OSRD S-l SECTION ON DECEMBER 18, 1941
5.10. On December 18, 1941, a meeting of the reorganized S-l
Section was held. Conant was present and discussed the new
policy, which called for an all-out effort. He emphasized that
such an effort was justified only by the military value of atomic
bombs and that all attention must be concentrated in the direc-
tion of bomb development. The whole meeting was pervaded
by an atmosphere of enthusiasm and urgency. Several methods
of electromagnetic separation were proposed and discussed, and
a number of new contracts were recommended.
MEETING OF OSRD S-l SECTION ON JANUARY 16, 1942
5.11. Another meeting of the OSRD S-l Section was held on
January 16, 1942. Informal discussions of the various production
Administrative History 1942-1945 79
methods took place, and tentative estimates were made as to
when each method would produce results. These forecasts
actually were no more than guesses since at that time the scientific
information available was very incomplete and the problems of
applying such data as did exist to the construction and operation
of production plants had hardly been approached.
REARRANGEMENT OF THE WORK EARLY IN 1942
5.12. In the middle of January 1942, Compton decided to
concentrate the work for which he was responsible at the Uni-
versity of Chicago. The Columbia group under Fermi and its
accumulated material and equipment and the Princeton group
which had been studying resonance absorption were moved to
Chicago in the course of the spring. Certain smaller groups else-
where remained active under Compton's direction. Under
Lawrence the investigation of large-scale electromagnetic
separation was accelerated at the University of California at
Berkeley and a related separation project was started at Princeton.
Research and development on the diffusion process and on the
production of heavy water continued at Columbia under Urey;
under the general supervision of Murphree, the centrifuge work
continued at the University of Virginia under Beams while the
Columbia centrifuge work was transferred to the laboratories of
the Standard Oil Development Co. at Bayway, New Jersey.
REPORT TO THE PRESIDENT BY BUSH
ON MARCH 9, 1942
5.13. In a report dated February 20, 1942, Conant recom-
mended that all phases of the work be pushed at least until July
1, 1942. Similarly, on March 9, 1942, Dr. Bush sent a report to
the President reflecting general optimism but placing proper
emphasis on the tentative nature of conclusions. His report
contemplated completion of the project in 1 944. In addition, the
report contained the suggestion that the Army be brought in
during the summer of 1 942 for construction of full-scale plants.
80 Administrative History 1942-1945
REVIEWS OF THE PROGRAM BY CONANT
5.14. The entire heavy-water program was under review in
March and April 1942. The reviews followed a visit to the United
States in February and March 1942 by F. Simon, H. Halban,
and W. A. Akers from England. In a memorandum of April 1,
1942 addressed to Bush, Conant reviewed the situation and re-
ported on conferences with Compton and Briggs. His report
pointed out that extremely large quantities of heavy water
would be required for a plutonium production plant employing
heavy water instead of graphite as a moderator. For this reason,
he reported adversely on the suggestion that Halban be invited
to bring to this country the 165 liters of heavy water which he then
had in England.
5.15. In a memorandum written to Bush on May 14, 1942
(shortly before a proposed meeting of Program Chiefs), Conant
estimated that there were five separation or production methods
which were about equally likely to succeed: the centrifuge, dif-
fusion, and electromagnetic methods of separating U-235; the
uranium-graphite pile and the uranium-heavy-water pile methods
of producing plutonium. All were considered about ready for
pilot plant construction and perhaps even for preliminary design
of production plants. If the methods were to be pushed to the
production stage, a commitment of five hundred million dollars
would be entailed. Although it was too early to estimate the
relative merits of the different methods accurately, it was pre-
sumed that some methods would prove to be more rapid and
efficient than others. It was feared, however, that elimination
of any one method might result in a serious delay. It was thought
that the Germans might be some distance ahead of the United
States in a similar program.
5.16. Conant emphasized a question that has been crucial
throughout the development of the uranium project. The ques-
tion was whether atomic bombs would be decisive weapons or
merely supplementary weapons. If they were decisive, there was
virtually no limit to the amount of effort and money that should
Administrative History 1942-1945 81
be put into the work. But no one knew how effective the atomic
bombs would be.
CHANGE FROM OSRD S-l SECTION TO OSRD S-l
EXECUTIVE COMMITTEE
5.17. In May 1942, Conant suggested to Bush that instead of
encouraging members of the section individually to discuss their
own phases of the work with Conant and Briggs, the OSRD S-l
Section should meet for general discussions of the entire program.
Bush responded by terminating the OSRD S-l Section and
replacing it with the OSRD S-l Executive Committee, consisting
of the following: J. B. Conant, chairman, L. J. Briggs, A. H.
Compton, E. O. Lawrence, E. V. Murphree, H. C. Urey. H. T.
Wensel and I. Stewart were selected to sit with the Committee
as technical aide and secretary respectively.
5.18. The following members of the old OSRD S-l Section
were appointed as consultants to the new Committee: S. K.
Allison, J. W. Beams, G. Breit, E. U. Condon, H. D. Smyth.
5.19. The functions of the new OSRD S-l Executive Com-
mittee were: (a) To report on the program and budget for the
next eighteen months, for each method, (b) To prepare recom-
mendations as to how many programs should be continued.
(c) To prepare recommendations as to what parts of the program
should be eliminated.
5.20. Recommendations relative to matters of OSRD S-l
policy and relative to the letting of OSRD S-l contracts were
made on the basis of a majority vote of the Committee. Conant
refrained from voting except in case of a tie vote. While Bush
alone had the authority to establish OSRD policies and commit
OSRD funds, he ordinarily followed the recommendations of
the S-l Executive Committee.
REPORT TO THE PRESIDENT BY BUSH AND CONANT
ON JUNE 17, 1942
5.21. On June 13, 1942, Bush and Conant sent to Vice-
President Henry A. Wallace, Secretary of War Henry L. Stimson,
82 Administrative History 1942-1945
and Chief of Staff General George C. Marshall a report recom-
mending detailed plans for the expansion and continuation of
the atomic-bomb program. All three approved the report. On
June 17, 1942, the report was sent by Bush to the President, who
also approved. The report, w r hich is too long to present in full,
contained four principal parts, which dealt with: (a) The status
of the development as appraised by the senior scientists; (b)
Recommendations by the program chiefs and Planning Board;
(c) Comments by Bush, Conant, and Maj. Gen. W. D. Styer; (d)
Recommendations by Bush and Conant. We may paraphrase
parts (a) and (c) as follows:
(a) The status of the program. (1) It was clear that an amount of
U-235 or plutonium comprising a number of kilograms would be
explosive, that such an explosion would be equivalent to several
thousand tons of TNT, and that such an explosion could be
caused to occur at the desired instant. (2) It was clear that there
were four methods of preparing the fissionable material and that
all of these methods appeared feasible; but it was not possible to
state definitely that any given one of these is superior to the others.
(3) It was clear that production plants of considerable size could
be designed and built. (4) It seemed likely that, granted adequate
funds and priorities, full-scale plant operation could be started
soon enough to be of military significance.
(c) Comments by Bush, Conant , and General Styer. Certain recom-
mendations had been made by Lawrence, Urey, Compton, and
Murphree. These recommendations had been reviewed by Bush,
Conant, and General Styer (who was instructed by General
Marshall to follow the progress of the program) and their com-
ments concerning the program were as follows: (1) If four sepa-
rate methods all appeared to a highly competent scientific group
to be capable of successful application, it appeared certain that
the desired end result could be attained by the enemy, provided
he had sufficient time. (2) The program as proposed obviously
could not be carried out rapidly without interfering with other
important matters, as regards both scientific personnel and
critical materials. A choice had to be made between the military
Administrative History 1942-1945 83
result which appeared attainable and the certain interference
with other war activities. (3) It was unsafe at that time, in view
of the pioneering nature of the entire effort, to concentrate on
only one means of obtaining the result. (4) It therefore appeared
best to proceed at once with those phases of the program which
interfered least with other important war activities. Work on
other phases of the program could proceed after questions of
interference were resolved.
5.22. The June 13, 1942, report to the President and Bush's
transmittal letter dated June 17, 1942, were returned to Bush
with the initialled approval of the President. A copy of the report
was then sent by Bush to General Styer on June 19, 1942.
SELECTION OF COLONEL J. C. MARSHALL
5.23. On June 18, 1942, Colonel J. C. Marshall, Corps of
Engineers, was instructed by the Chief of Engineers to form a
new district in the Corps of Engineers to carry on special work
(atomic bombs) assigned to it. This district was designated the
Manhattan District and was officially established on August 13,
1942. The work with which it was concerned was labeled, for
security reasons, the "DSM Project" (Development of Substitute
Materials).
SELECTION OF GENERAL L. R. GROVES
5.24. On September 17, 1942, the Secretary of War placed
Brigadier General L. R. Groves of the Corps of Engineers in
complete charge of all Army activities relating to the DSM
Project.
MILITARY POLICY COMMITTEE; FUNCTIONING OF
THE OSRD COMMITTEES
5.25. A conference was held on September 23, 1942, among
those persons designated by the President to determine the
general policies of the project, and certain others. Those present
were Secretary of War Henry L. Stimson, Chief of Staff General
84 Administrative History 1942-1945
George C. Marshall, Dr. J. B. Conant, Dr. V. Bush, Major
General Brehon Somervell, Major General W. D. Styer, and
Brigadier General L. R. Groves. (Vice-President Henry A.
Wallace was unable to attend.) A Military Policy Committee
was appointed consisting of Dr. V. Bush as Chairman with Dr.
J. B. Conant as his alternate, Major General W. D. Styer, and
Rear Admiral W. R. Purnell. General Groves was named to sit
with the committee and act as Executive Officer to carry out the
policies that were determined. The duties of this committee were
to plan military policies relating to materials, research and
development, production, strategy, and tactics, and to submit
progress reports to the policy group designated by the President.
5.26. The appointment of the Military Policy Committee was
approved by the Joint New Weapons Committee, established by
the U. S. Joint Chiefs of Staff and consisting of Dr. V. Bush, Rear
Admiral W. R. Purnell, and Brigadier General R. G. Moses.
5.27. The creation of the Military Policy Committee in effect
placed all phases of the DSM Project under the control of Dr.
Bush, Dr. Conant, General Styer, Admiral Purnell, and General
Groves.
5.28. The OSRD S-l Executive Committee held meetings
about once every month from June 1 942 to May 1 943 and once
after that time, in September 1943. These meetings were normally
attended by General Groves, after September 1942, and Colonel
Marshall, and frequently by representatives of the industrial
companies concerned with the production plants. Recommenda-
tions of the Committee were not binding but were usually
followed. Thus it served as an advisory body to Dr. Bush and
General Groves, and as an initial liaison group between the
scientific, industrial, and military parts of the DMS Project.
The S-l Executive Committee has never been formally dissolved,
but it has been inactive since the fall of 1943.
5.29. The procurement and engineering functions of the
Planning Board were taken over by the Manhattan District in
the summer of 1942 and that board then became inactive.
5.30. By the spring of 1943 it was felt that the Manhattan
Administrative History 1942-1945 85
District was in a position to take over research and development
contracts from the OSRD. Such a transfer was effected as of
May 1, 1943, and marked the end of the formal connection of
OSRD with the uranium project.
5.31. In July 1943 Conant and R. G. Tolman were formally
asked by General Groves to serve as his scientific advisers. They
had already been doing so informally and have continued to do
so. Coordination of the various scientific and technical programs
was accomplished by meetings between General Groves and the
leaders of the various projects, in particular, Compton, Lawrence,
Oppenheimer (see Ghapter XII), and Urey.
SUBSEQUENT ORGANIZATION: THE
MANHATTAN DISTRICT
5.32. Since 1943 there have been no important changes in
the form of the organization and few of importance in the operat-
ing personnel. General Groves has continued to carry the major
responsibility for correlating the whole effort and keeping it
directed toward its military objectives. It has been his duty to
keep the various parts of the project in step, to see that raw
materials were available for the various plants, to determine
production schedules, to make sure that the development of
bomb design kept up with production schedules, to arrange for
use of the bombs when the time came, and to maintain an ade-
quate system of security. In discharging these duties General
Groves has had the help of his tremendous organization made up
of civilian scientists and engineers and Engineer officers and
enlisted men. Many of the civilians have been mentioned already
or will be mentioned in later chapters dealing with particular
projects. Brigadier General T. F. Farrell has acted as General
Groves' deputy in the important later phases of the project.
Colonel K. D. Nichols, the District Engineer of the Manhattan
District with his headquarters at the Clinton Engineer Works, has
been connected with the project since 1942. He has been con-
cerned with the research and production problems of both U-235
86 Administrative History 1942-1945
and plutonium and has always shown exceptional understanding
of the technical problems and their relative importance. Two
other officers who should be mentioned are Colonel F. T. Mat-
thias and Colonel S. L. Warren. Colonel Matthias has discharged
major responsibilities at the Hanford Engineer Works in an
extremely able manner; his duties have been concerned with
both the construction and operational phases of the project.
Colonel Warren is chief of the Medical Section of the Manhattan
District and therefore has had ultimate responsibility for health
problems in all parts of the project.
SUMMARY
5.33. By the end of 1941 an extensive review of the whole
uranium situation had been completed. As a result of this review
Bush and his advisers decided to increase the effort on the
uranium project and to change the organization. This decision
was approved by President Roosevelt. From January 1 942 until
early summer of 1942 the uranium work was directed by Bush
and Conant working with the Program Chiefs and a Planning
Board. In the summer of 1942 the Army, through the Corps of
Engineers, was assigned an active part in the procurement and
engineering phases, organizing the Manhattan District for the
purpose. In September 1942, Dr. Bush, Dr. Conant, General
Styer, and Admiral Purnell were appointed as a Military Policy
Committee to determine the general policies of the whole project.
Also in September, General Groves was appointed to take charge
of all Army activities of the project. The period of joint OSRD
and Army control continued through April 1943 with the Army
playing an increasingly important role as the industrial effort got
fully under way. In May 1943 the research contracts were trans-
ferred to the Corps of Engineers; the period of joint OSRD- Army
control ended and the period of complete Army control began.
5.34. Since the earliest days of the project, President Roosevelt
had followed it with interest and, until his death, continued
to study and approve the broad programs of the Military Policy
Administrative History 1942-1945 87
Committee. President Truman, who as a United States Senator
had been aware of the project and its magnitude, was given the
complete up-to-date picture by the Secretary of War and General
Groves at a White House conference immediately after his
inauguration. Thereafter the President gave the program his
complete support, keeping in constant touch with the progress.
CHAPTER VI. THE METALLURGICAL PROJECT
AT CHICAGO IN 1942
INTRODUCTION
6.1. As has been made clear in Chapters IV and V, the infor-
mation accumulated by the end of 1941 as to the possibility of
producing an atomic bomb was such as to warrant expansion of
the w r ork, and this expansion called for an administrative reor-
ganization. It was generally accepted that there was a very high
probability that an atomic bomb of enormous destructive power
could be made, either from concentrated U-235 or from the new
element plutonium. It was proposed, therefore, to institute an
intensive experimental and theoretical program including work
both on isotope separation and on the chain-reaction problems.
It was hoped that this program would establish definitely whether
or not U-235 could be separated in significant quantities from
U-238, either by electromagnetic or statistical methods; whether
or not a chain reaction could be established with natural uranium
or its compounds and could be made to yield relatively large
quantities of plutonium; and whether or not the plutonium so
produced could be separated from the parent material, uranium.
It was hoped also that the program would provide the theo-
retical and experimental data required for the design of a fast-
neutron chain-reacting bomb.
6.2. As has been explained in Chapter V, the problems of
isotope separation had been assigned to groups under Lawrence
and Urey while the remaining problems were assigned to Comp-
ton's group, which was organized under the cryptically named
"Metallurgical Laboratory" of the University of Chicago. In
this chapter and the following two chapters we shall describe the
work of the Metallurgical Laboratory and the associated labora-
tories up to June 1945. In later chapters we shall discuss isotope-
88
Metallurgical Project in 1942 89
separation work and the work of the bomb development group,
which was separated from the Metallurgical Laboratory early
in 1943.
6.3. It would be futile to attempt an assessment of the relative
importance of the contributions of the various laboratories to the
overall success of the atomic-bomb project. This report makes no
such attempt, and there is little correlation between the space
devoted to the work of a given group and the ability or importance
of that group. In deciding which subdivision of the atomic-bomb
project should be discussed first and most fully, we have been
governed by criteria of general interest and of military security.
Some developments of great technical importance are of little
ueneral interest; others both interesting and important must still
be kept secret. Such criteria, applied to the objectives and ac-
complishments of the various laboratories set up since large-scale
work began, favor the Metallurgical Laboratory as the part of
the project to be treated most completely.
OBJECTIVES
6.4. In accordance with the general objectives just outlined,
the initial objectives of the Metallurgical Laboratory were: first,
to find a system using normal uranium in which a chain reaction
would occur; second, to show that, if such a chain reaction did
occur, it would be possible to separate plutonium chemically from
the other material; and, finally, to obtain the theoretical and
experimental data for effecting an explosive chain reaction with
either U-235 or with plutonium. The ultimate objective of the
laboratory was to prepare plans for the large-scale production of
plutonium and for its use in bombs.
ORGANIZATION OF THE WORK
6.5. The laboratory had not only to concern itself with its
immediate objectives but simultaneously to bear in mind the
ultimate objectives and to work toward them on the assumption
that the immediate objectives would be attained. It could not
wait for a chain reaction to be achieved before studying the
90 Metallurgical Project in 1942
chemistry of plutonium. It had to assume that plutonium would
be separated and to go ahead with the formulation of plans for
its production and use. Consequently problems were continually
redefined as new information became available, and research
programs were reassessed almost from week to week. In a general
way the experimental nuclear physics group under E. Fermi was
primarily concerned with getting a chain reaction going, the
chemistry division organized by F. H. Spedding (later in turn
under S. K. Allison, J. Franck, W. C. Johnson, and T. Hogness)
with the chemistry of plutonium and with separation methods,
and the theoretical group under E. Wigner with designing pro-
duction piles. However, the problems were intertwined and the
various scientific and technical aspects of the fission process were
studied in whatever group seemed best equipped for the particu-
lar task. In March 1942, Thomas Moore was brought in to
head the engineering group. Other senior men in this group were
M. C. Leverett, J. A. Wheeler and C. M. Cooper, who later
succeeded Moore as head of the Technical Division. In the
summer of 1942 the importance of health problems became
apparent and a Health Division was organized under Dr. R. S.
Stone. The difficult task of organizing and administering a re-
search laboratory growing in size and complexity with almost
explosive violence was carried out by R. L. Doan as Laboratory
Director.
6.6. We have chosen to confine this chapter to the work of
1942 because a self-sustaining chain reaction was first achieved
on December 2 of that year, at a time when the whole Chicago
project was being appraised by a reviewing committee with the
members particularly selected for their engineering background. *
That was a dramatic coincidence and also a convenient one for
purposes of this report since either incident might be considered
to mark the end of an epoch at the Metallurgical Laboratory.
Furthermore, in preparation for the reviewing committee's visit
* This committee was composed of W. K. Lewis, C. H. Greenewalt,
T. C. Gary, and Roger Williams. E. V. Murphree was also a member but
due to illness was unable to participate.
Metallurgical Project in 1942 91
a comprehensive report had been prepared. That report was
generally known as the "Feasibility Report" and has been used
extensively in preparing this chapter.
PLAN OF THIS CHAPTER
6.7. In this chapter we shall present the material in the order
of the objectives given above. In Part I we shall discuss progress
towards the initial objectives, including (a) procurement of
materials, (b) the experimental proof of the chain reaction, (c) the
chemistry of plutonium and some of the problems of separation,
(d) some of the types of auxiliary experiments that were per-
formed, and finally (e) the "fast neutron" work. Necessarily
the work described in detail is only a sampling of the large amount
of theoretical and experimental work actually performed. In
Part II we shall discuss the possibilities that were considered for
production piles and separation methods, and the specific pro-
posals made in November 1942.
PART I: PROGRESS TOWARD THE
INITIAL OBJECTIVES
PROCUREMENT OF MATERIALS
GENERAL
6.8. It has been made clear in earlier chapters of this report
that the procurement of materials of sufficient purity was a major
part of the problem. As far as uranium was concerned, it seemed
likely that it would be needed in highly purified metallic form
or at least as highly purified uranium oxide. The other materials
which were going to be needed were either graphite, heavy water,
or possibly beryllium. It was clear at this time that, however
advantageous heavy water might be as a moderator, no large
quantities of it would be available for months or years. Beryllium
seemed less advantageous and almost as difficult to get. There-
fore the procurement efforts for a moderator were centered on
graphite. As has been explained in Chapter V, procurement of
92 Metallurgical Project in 1942
uranium and graphite was not primarily the responsibility of
the Metallurgical Laboratory but was handled through E. V.
Murphree and others on the "planning board." In fact, the
obvious interest of the Metallurgical Laboratory in the problem
led to continual intervention by its representatives. A great deal
of the credit for the eventual success in obtaining materials is due
to N. Hilberry and later R. L. Doan, always supported by
A. H. Compton.
URANIUM ORE
6.9. Obviously there would be no point in undertaking this
whole project if it were not going to be possible to find enough
uranium for producing the bombs. Early indications were favor-
able, and a careful survey made in November 1942 showed that
immediate delivery could be made of adequate tonnages of
uranium ores.
URANIUM OXIDE AND URANIUM METAL
6.10. At the end of 1941 the only uranium metal in existence
was a few grams of good material made on an experimental basis
by the Westinghouse Electric and Manufacturing Company and
others and a few pounds of highly impure pyrophoric powder
made by Metal Hydrides Company. The only considerable
amount of raw material then available in this country was in the
form of a commercial grade of black uranium oxide, which could
be obtained in limited quantities from the Canadian Radium
and Uranium Co. It contained 2 to 5 per cent of impurities and
was the material which gave a neutron multiplication factor of
only about 0.87 when used in an exponential pile.
6.11. By May 1942, deliveries averaging 15 tons a month of
black oxide of higher purity and more uniform grade started
coming in. Total impurities were less than 1 per cent, boron
comprised a few parts per million, and the neutron multiplication
factor (k) was about 0.98. (It is to be remembered that the
multiplication factor depends also on the purity of the graphite.)
Deliveries of this material reached a ton a day in September 1942.
Metallurgical Project in 1942 93
6.12. Experiments at the National Bureau of Standards by
J. I. Hoffman demonstrated that, by the use of an ether extraction
method, all the impurities are removed by a single extraction of
uranyl nitrate. The use of this method removed the great bulk
of the difficulties in securing pure oxide and pure materials for
the production of metal. Early in May 1942, arrangements were
completed with the Mallinckrodt Chemical Works in St. Louis
to put the new grade of oxide through an ether extraction process
on a production basis for a further reduction in impurity content
and to deliver the final product as brown dioxide. Deliveries
started in July 1942 at a rate of 30 tons a month. This oxide is
now used as a starting point for all metal production, and no
higher degree of purity can be expected on a commercial scale.
In fact, it was a remarkable achievement to have developed and
put into production on a scale of the order of one ton per day a
process for transforming grossly impure commercial oxide to
oxide of a degree of purity seldom achieved even on a laboratory
scale.
6.13. The process which Westinghouse had been using to pro-
duce the metal was the electrolysis of KUFs at a cost of about
$1,000 a pound. Since the KUFs was produced photochemically
under the action of sunlight this method constituted a potential
bottleneck in production. It was found that uranium tetrafluoride
could be used instead of KUFs, and steps were taken to have this
salt produced at the Harshaw Chemical Company in Cleveland
and at the du Pont plant in Penns Grove, New Jersey. Production
started in August 1942 and by October 1942 was up to 700
pounds per day at Harshaw and 300 pounds per day at du Pont,
the method of manufacture in both cases being the hydrofluorina-
tion of Mallinckrodt-purified dioxide.
6.14. As the result of this supply of raw materials to Westing-
house, and as a result of plant expansion, deliveries from Westing-
house had accumulated to a total of more than 6,000 pounds by
November 1 942 and were expected to be at the rate of 500 pounds
per day by January 1943. The purity of the metal was good, and
the cost had dropped to $22 per pound.
94 Metallurgical Project in 1942
6.15. Deliveries of acceptable metal from Metal Hydrides Co.
were delayed for various reasons and were just beginning in
November 1942. This company's production was supposed to
reach a thousand pounds per week thereafter.
6.16. Neither the Westinghouse process nor the Metal Hy-
drides Process was entirely satisfactory. Intensive activity designed
to accelerate metal production, and carried out independently
by F. H. Spedding and his associates at Iowa State College at
Ames, Iowa, and by C. J. Rodden at the National Bureau of
Standards, resulted in the development of a satisfactory method.
Production facilities were set up at Ames in the fall of 1942 and
had already produced more than one ton by the end of November.
The process was extremely simple, rapid and low cost.
6.17. Further research indicated additional changes that could
be made to advantage, and by the middle of 1943 Spedding at
Iowa and other producers who entered the picture were using
the final production method adopted.
6.18. By the end of 1942 arrangements had been made by the
Manhattan District to increase metal production by making
greater use of the Mallinckrodt Chemical Works, the Union
Carbide and Carbon Corporation, and the du Pont Company.
6.19. To summarize, almost no metal was available during
most of 1942, a fact that seriously delayed progress as we shall
see, but the production problems had been nearly solved by the
end of 1 942 and some 6 tons of metal were incorporated in the
pile built in November 1942. The whole problem of procurement
of metal was taken over by the Manhattan District at the end
of the year, under the general direction of Colonel RuhofT,
formerly with the Mallinckrodt Chemical Works. From the point
of view of the Metallurgical Project no further serious delays or
difficulty have occurred because of metal shortages.
GRAPHITE PROCUREMENT
6.20. At the beginning of 1942 graphite production was still
unsatisfactory but it was, of course, in quite a different condition
from the metal production since the industrial production of
Metallurgical Project in 1942 95
graphite had already been very large. The problem was merely
one of purity and priority. Largely through the efforts of N. Hil-
berry, the National Carbon Company and the Speer Carbon
Company were both drawn into the picture. Following sugges-
tions made by the experts of the National Bureau of Standards,
these companies were able to produce highly purified graphite
with a neutron absorption some 20 per cent less than the standard
commercial materials previously used. Although efforts further to
reduce the impurities have had some success, the purity problem
was essentially solved by the middle of 1942 and large orders
were placed with the cooperation of the War Production Board.
As in the case of the metal, the graphite procurement problem
was taken over by the Manhattan District.
THE CHAIN REACTION
FURTHER INTERMEDIATE EXPERIMENTS
6.21. At the time that the Metallurgical Project was organized,
most of the physicists familiar with the problem believed that a
chain-reacting pile probably could be built if sufficiently pure
graphite and pure uranium metal could be obtained. Enough
work had been done on resonance absorption, on the theory of
absorption and diffusion of neutrons hi a pile, and on inter-
mediate experiments to make it possible to design a lattice struc-
ture that had a very good chance of maintaining a chain reaction.
Nevertheless, there were uncertainties in the experimental data
and in the approximations that had to be made in the theoretical
calculations. There were two alternatives: (1) to build a pile
according to the best possible design; (2) to make more accurate
determinations of the pertinent nuclear constants, to perform
intermediate experiments, and to improve the calculations. There
is little doubt that the first alternative was the one likely to lead
most rapidly to the production of plutonium. There were many
important questions which could have been answered more
rapidly by such an operating pile than by a series of small-scale
experiments. Unfortunately, the necessary amounts of materials
96 Metallurgical Project in 1Q42
were not available and did not become available for nearly nine
months. Consequently, it was necessary to choose the second
alternative, that is, to accumulate all relevant or possibly relevant
information by whatever means were available.
6.22. The major line of investigation was a series of inter-
mediate experiments. The particular set-up for each intermediate
experiment could be used to test calculations based on separate
auxiliary experiments. For example, the proportion of uranium
oxide to graphite was varied, oxides of different purities were
used, oxide was used in lumps of various sizes and shapes and
degrees of compression, the lattice spacing was varied, the effect
of surrounding the uranium oxide units with beryllium and with
paraffin was tried; and, finally, piles of identical lattice type but
of different total size were tried to see whether the values of the
multiplication factor k (for infinite size) calculated from the
different sets of results were identical. In general, E. Fermi had
direct charge of investigations of effects of impurities, and S. K.
Allison had charge of tests involving different lattice dimensions.
All these experiments strengthened the confidence of the group
in the calculated value of k and in the belief that a pile could be
built with k greater than unity. In July enough purified uranium
oxide from Mallinckrodt was available to permit building inter-
mediate pile No. 9. As in previous experiments, a radium-
beryllium neutron source was placed at the bottom of the lattice
structure and the neutron density measured along the vertical
axis of the pile. By this time it was known that the neutron density
decreased exponentially with increasing distance from the neutron
source (hence the name often used for experiments of this type,
"exponential pile") and that, from such rates of decrease, the
multiplication constant k for an infinitely large pile of the same
lattice proportions could be calculated. For the first time the
multiplication constant k so calculated from experimental results
came out greater than one. (The actual value was 1.007.) Even
before this experiment Compton predicted in his report of July 1
that a k value somewhere between 1.04 and 1.05 could be ob-
tained in a pile containing highly purified uranium oxide and
Metallurgical Project in 1942 97
graphite, provided that the air was removed from the pile to
avoid neutron absorption by nitrogen.
AN AUXILIARY EXPERIMENT; DELAYED NEUTRONS
6.23. We shall not mention a majority of the various auxiliary
experiments done during this period. There was one, however,
the study of delayed neutrons that we shall discuss because it is
a good example of the kind of experiment that had to be per-
formed and because it concerned one effect, not heretofore men-
tioned, that is of great importance in controlling a chain-reacting
pile.
6.24. From previous investigations, some of which were already
published, it was known that about 1 per cent of the neutrons
emitted in fission processes were not ejected immediately but
were given off in decreasing quantity over a period of time, a
fact reminiscent of the emission of beta rays from shortlived
radioactive substances. Several half-lives had been observed, the
longest being of the order of a minute.
6.25. It was realized early that this time delay gave a sort of
inertia to the chain reaction that should greatly facilitate control.
If the effective multiplication factor of a pile became slightly
greater than 1, the neutron density would not rise to harmfully
large values almost instantly but would rise gradually so that
there would be a chance for controls to operate. (Other time
intervals involved, such as those between collisions, are too small
to be useful.)
6.26. Because of the importance of this effect of delayed neu-
trons for control it was decided to repeat and improve the earlier
measurements. (The fact that this was a repetition rather than a
new measurement is also typical of much of the work in physics
at this period.) A description of the experiment is given in Ap-
pendix 3. The results indicated that 1.0 per cent of the neutrons
emitted in uranium fission are delayed by at least 0.01 second
and that about 0.7 per cent are delayed by as much as a minute.
By designing a pile such that the effective value of k, the multi-
98 Metallurgical Project in 1942
plication factor, is only 1.01 the number of delayed neutrons is
sufficient to allow easy control.
THE FIRST SELF-SUSTAINING CHAIN-REACTING PILE
6.27. By the fall of 1942 enough graphite, uranium oxide, and
uranium metal were available at Chicago to justify an attempt
to build an actual self-sustaining chain-reacting pile. But the
amount of metal available was small only about 6 tons and
other materials were none too plentiful and of varying quality.
These conditions rather than optimum efficiency controlled the
design.
6.28. The pile was constructed on the lattice principle with
graphite as a moderator and lumps of metal or oxide as the
reacting units regularly spaced through the graphite to form the
lattice. Instruments situated at various points in the pile or near
it indicated the neutron intensity, and movable strips of absorbing
material served as controls. (For a more complete description of
the pile, see Appendix 4.) Since there were bound to be some
neutrons present from spontaneous fission or other sources, it
was anticipated that the reaction would start as soon as the struc-
ture had reached critical size if the control strips were not set
in "retard" position. Consequently, the control strips were placed
in a suitable "retard" position from the start and the neutron
intensity was measured frequently. This was fortunate since the
approach to critical condition was found to occur at an earlier
stage of assembly than had been anticipated.
6.29. The pile was first operated as a self-sustaining system on
December 2, 1942. So far as we know, this was the first time that
human beings ever initiated a self-maintaining nuclear chain
reaction. Initially the pile was operated at a power level of
J^ watt, but on December 12 the power level was raised to 200
watts.
ENERGY DEVELOPED BY THE PILE
6.30. In these experiments no direct measurements of energy
release were made. The number of neutrons per second emitted
Metallurgical Project in 1942 99
by the pile was estimated in terms of the activity of standardized
indium foils. Then, from a knowledge of the number of neutrons
produced per fission, the resultant rate of energy release (wattage)
was calculated.
CONCLUSION?
6.31. Evidently this experiment, performed on December 2
just as a reviewing committee was appraising the Chicago project,
answered beyond all shadow of doubt the first question before the
Metallurgical Laboratory; a self-sustaining nuclear chain reac-
tion had been produced in a system using normal uranium. This
experiment had been performed under the general direction of
E. Fermi, assisted principally by the groups headed by W. H. Zinn
and H. L. Anderson. V. C. Wilson and his group had been largely
responsible for developing the instruments and controls, and a
great many others in the laboratory had contributed to the
success of the enterprise.
RELATION BETWEEN POWER AND PRODUCTION
OF PLUTONIUM
6.32. The immediate object of building a uranium-graphite
pile was to prove that there were conditions under which a chain
reaction would occur, but the ultimate objective of the laboratory
was to produce plutonium by a chain reaction. Therefore we are
interested in the relation between the power at which a pile
operates and the rate at which it produces plutonium. The rela-
tion may be evaluated to a first approximation rather easily.
A pile running stably must be producing as many neutrons as it
is losing. For every thermal neutron absorbed in U-235 a certain
number of neutrons, r;, is emitted. One of these neutrons is re-
quired to maintain the chain. Therefore, assuming the extra
neutrons all are absorbed by U-238 to form plutonium, there will
be TJ 1 atoms of Pu 239 formed for every fission. Every fission
releases roughly 200 Mev of energy. Therefore the formation
of 77 1 atoms of plutonium accompanies the release of about
200 Mev. Since 77 1 is a small number, we can guess that to
100 Metallurgical Project in 1942
produce a kilogram a day of plutonium a chain-reacting pile
must be releasing energy at the rate of 500,000 to 1,500,000 kilo-
watts. The first chain-reacting pile that we have just described
operated at a maximum of 200 watts. Assuming that a single
bomb will require the order of one to 100 kilograms of plutonium,
the pile that has been described would have to be kept going at
least 70,000 years to produce a single bomb. Evidently the prob-
lem of quantity production of plutonium was not yet solved.
THE CHEMISTRY OF PLUTONIUM
6.33. The second specific objective of the Metallurgical
Laboratory was to show that, if a chain reaction did occur, it
would be feasible to separate the plutonium chemically from the
other material with which it is found. Progress toward this objec-
tive was necessarily slower than toward the attainment of a chain
reaction. Initially little was done at the Metallurgical Laboratory
on chemical problems although the extraction problem was dis-
cussed in a conference soon after the project was organized and
the work of Seaborg's group at the University of California on
plutonium was encouraged. On April 22-23, 1942, a general
conference on chemistry was held at Chicago, attended by F. H.
Spedding, E. W. Thiele, G. T. Seaborg, J. W. Kennedy, H. C.
Urey, E. Wigner, N. Hilberry, G. E. Boyd, I. B.Johns, H. A. Wil-
helm, I. Perlman, A. C. Wahl, and J. A. Wheeler. Spedding, in
opening the meeting, pointed out that there were two main tasks
for the chemists: first, to separate plutonium in the amounts and
purity required for war purposes; second, to obtain a good under-
standing of the chemistry necessary for the construction and
maintenance of the pile. The separation problem was to be studied
by a new group at Chicago under the direction of Seaborg, by
Johns and Wilhelm at Ames, and by Wahl and Kennedy con-
tinuing the work at California. Other closely related groups at
Chicago were to be C. D. Coryell's, working on the fission prod-
ucts, and Boyd's on analytical problems. The chemistry group at
Chicago has grown speedily since that time. A new building had
to be constructed to house it late in 1942, and this building was
Metallurgical Project in 1942 101
enlarged subsequently. Altogether, the solving of many of the
chemical problems has been one of the most remarkable achieve-
ments of the Metallurgical Laboratory.
6.34. The first isotope of plutonium discovered and studied
was not the 239 isotope but the 238 isotope, which is an alpha-ray
emitter with a half-life of about 50 years. U-238 bombarded with
deuterons gives gsNp 238 which disintegrates to 94Pu 238 by beta
emission.* The first evidence of the actual existence of these new
elements (ruling out the original erroneous interpretation of the
splitting of uranium as evidence for their existence) was obtained
by E. McMillan and P. H. Abelson who isolated 93-238 from
uranium bombarded with deuterons in the Berkeley cyclotron.
This new element w r as identified as a beta emitter but the sample
was too small for isolation of the daughter product 94-238. Later,
enough Pu-238 was prepared to permit Seaborg, Kennedy and
\\ ahl to begin the study of its chemical properties in the winter
of 1940-1941 by using tracer chemistry with carriers according
to practice usual in radiochemistry. By such studies many
chemical properties of plutonium were determined, and several
possible chemical processes were evolved by which Pu-239 might
be removed from the chain-reacting pile. The success of experi-
ments on a tracer scale led to plans to produce enough Pu-239
to be treated as an ordinary substance on the ultra-microchemical
M-ale. Such quantities were produced by prolonged bombard-
ment of several hundred pounds of uranyl nitrate with neutrons
obtained with the aid of cyclotrons, first at Berkeley and later at
Washington University in St. Louis. By the end of 1942, some-
tiling over 500 micrograms had been obtained in the form of pure
plutonium salts. Although this amount is less than would be
needed to make the head of a pin, for the micro-chemists it was
sufficient to yield considerable information; for one microgram
is considered sufficient to carry out weighing experiments,
dtrations, solubility studies, etc.
6.35. From its position in the periodic table, plutonium might
l;>e expected to be similar to the rare earths or to uranium*
* See drawing on p. 8.
102 Metallurgical Project in 1942
thorium, or osmium. Which of these it will resemble most closely
depends, of course, on the arrangement of the outermost groups
of electrons and this arrangement could hardly have been pre-
dicted. On the whole, plutonium turned out to be more like
uranium than like any of the other elements named and might
even be regarded as the second member of a new rare-earth
series beginning with uranium. It was discovered fairly early
that there were at least two states of oxidation of plutonium.
(It is now known that there are four, corresponding to positive
valences of 3, 4, 5, and 6.) Successful microchemical preparation
of some plutonium salts and a study of their properties led to the
general conclusion that it was possible to separate plutonium
chemically from the other materials in the pile. This conclusion
represents the attainment of the second immediate objective of
the Metallurgical Laboratory. Thus, by the end of 1942,
plutonium, entirely unknown eighteen months earlier, was con-
sidered an element whose chemical behavior was as well under-
stood as that of several of the elements of the old periodic table.
MISCELLANEOUS STUDIES
6.36. Besides the major problems we have mentioned, i.e., the
chain reaction, the chemical separation, and the planning for a
production plant, there were innumerable minor problems to be
solved. Among the more important of these were the improve-
ment of neutron counters, ionization chambers, and other instru-
ments, the study of corrosion of uranium and aluminum by
water and other possible coolants, the determination of the
effects of temperature variation on neutron cross sections, the
fabrication of uranium rods and tubes, the study of fission prod-
ucts, and the determination of the biological effects of radiation.
As typical of this kind of work we can cite the development of
methods of fabricating and coating uranium metal, under the
direction of E. Creutz. Without the accomplishment of these
secondary investigations the project could not have reached its
goal. To give some further idea of the scope of the work, a list
Metallurgical Project in 1942 103
of twenty report titles is presented in Appendix 5, the 20 reports
being selected from the 400 or so issued during 1942.
THE FAST-NEUTRON REACTION
6.37. The third initial objective of the Metallurgical Project
was to obtain theoretical and experimental data on a "fast
neutron" reaction, such as would be required in an atomic
bomb. This aspect of the work was initially planned and coordi-
nated by G. Breit of the University of Wisconsin and later con-
tinued by J. R. Oppenheimer of the University of California.
Since the actual construction of the bomb was to be the final part
of the program, the urgency of studying such reactions was not
so great. Consequently, little attention was given to the theo-
retical problems until the summer of 1942, when a group was
organized at Chicago under the leadership of Oppenheimer.
6.38. In the meantime experimental work initiated in most
instances by G. Breit, had been in progress (under the general
direction of the Metallurgical Project) at various institutions
having equipment suitable for fast-neutron studies (Carnegie
Institution of Washington, the National Bureau of Standards,
Cornell University, Purdue University, University of Chicago,
University of Minnesota, University of Wisconsin, University of
California, Stanford University, University of Indiana, and Rice
Institute). The problems under investigation involved scattering,
absorption and fission cross section, the energy spectrum of
fission neutrons, and the time delay in the emission of fission
neutrons. For the most part this work represented an intermediate
step in confirming and extending previous measurements but
reached no new final conclusion. This type of work was subse-
quently concentrated at another site (see Chapter XII).
6.39. As indicated by the "Feasibility Report" (in a section
written by J. H. Manley, J. R. Oppenheimer, R. Serber, and
E. Teller) the picture had changed significantly in only one
respect since the appearance of the National Academy Report
a year earlier. Theoretical studies now showed that the effective-
ness of the atomic bomb in producing damage would be greater
104 Metallurgical Project in 1942
than had been indicated in the National Academy report. How-
ever, critical size of the bomb was still unknown. Methods of
detonating the bomb had been investigated somewhat, but on
the whole no certain answers had been reached.
PART II: PROGRESS TOWARD THE
ULTIMATE OBJECTIVE
PLANNING A PRODUCTION PLANT
PLANNING AND TECHNICAL WORK
6.40. As we have seen, the initial objectives of the Metallurgical
Laboratory had been reached by the end of 1942, but the ulti-
mate objectives, the production of large quantities of plutonium
and the design and fabrication of bombs, were still far from at-
tained. The responsibility for the design and fabrication of bombs
was transferred to another group at about this time; its work is
reported in Chapter XII. The production of Pu-239 in quantity
has remained the principal responsibility of the Metallurgical
Laboratory although shared with the du Pont Company since
the end of 1942.
6.41 . On the basis of the evidence available it was clear that
a plutonium production rate somewhere between a kilogram a
month and a kilogram a day would be required. At the rate of a
kilogram a day. a 500,000 to 1,500,000 kilowatt plant would be
required. (The ultimate capacity of the hydroelectric power
plants at the Grand Coulee Dam is expected to be 2,000,000 kw.)
Evidently the creation of a plutonium production plant of the
required size was to be a major enterprise even without attempt-
ing to utilize the thermal energy liberated. Nevertheless, by
November 1942 most of the problems had been well defined and
tentative solutions had been proposed. Although these problems
will be discussed in some detail in the next chapter, we will
mention them here.
6.42. Since a large amount of heat is generated in any pile
producing appreciable amounts of plutonium, the first problem
Metallurgical Project in 1942 105
of design is a cooling system. Before such a system can be de-
signed, it is necessary to find the maximum temperature at which
a pile can run safely and the factors nuclear or structural
\\ hich determine this temperature. Another major problem is the
method for loading and unloading the uranium, a problem com-
plicated by the shielding and the cooling system. Shielding against
radiation has to be planned for both the pile itself and the chemi-
cal separation plant. The nature of the separation plant depends
on the particular separation process to be used, which has to be
decided. Finally, speed of procurement and construction must
be primary factors in the planning of both the pile and the
chemical plant.
POSSIBLE TYPES OF PLANT
6.43. After examining the principal factors affecting plant
design, i.e., cooling, efficiency, safety, and speed of construction,
the "Feasibility Report" suggested a number of possible plant
types in the following order of preference:
I. (a) Ordinary uranium metal lattice in a graphite moder-
ator with helium cooling, (b) The same, with water
cooling, (c) The same, with molten bismuth cooling.
II. Ordinary uranium metal lattice in a heavy-water moder-
ator.
III. Uranium enriched in the 235 isotope using graphite,
heavy water, or ordinary water as moderator.
Types II and III were of no immediate interest since neither
enriched uranium nor heavy water was available. Development
of both these types continued however, since if no other type
proved feasible they might have to be used. Type I (c), calling
for liquid bismuth cooling, seemed very promising from the point
of view of utilization of the thermal energy released, but it was
felt that the technical problems involved could not be solved for
a long time.
106 Metallurgical Project in 1942
THE PILOT PLANT AT CLINTON
6.44. During this period, the latter half of 1942, when pro-
duction plants were being planned, it was recognized that a plant
of intermediate size was desirable. Such a plant was needed for
two reasons: first, as a pilot plant; second, as a producer of a few
grams of plutonium badly needed for experimental purposes.
Designed as an air-cooled plant of 1,000-kw capacity, the inter-
mediate pile constructed at Clinton, Tennessee, might have
served both purposes if helium cooling had been retained for the
main plant. Although the plans for the main plant were shifted
so that water cooling was called for, the pilot plant was continued
with air cooling in the belief that the second objective would be
reached more quickly. It thus ceased to be a pilot plant except
for chemical separation. Actually the main plant was built
without benefit of a true pilot plant, much as if the hydroelectric
generators at Grand Coulee had been designed merely from
experience gained with a generator of quite different type and
of a small fraction of the power.
SPECIFIC PROPOSALS
6.45. As reviewed by Hilberry in the "Feasibility Report" of
November 26, 1 942, the prospects for a graphite pile with helium
cooling looked promising as regards immediate production; the
pile using heavy water for moderator and using heavy water or
ordinary water as coolant looked better for eventual full-scale
use. A number of specific proposals were made for construction
of such plants and for the further study of the problems involved.
These proposals were based on time and cost estimates which were
necessarily little better than rough guesses. As the result of further
investigation the actual program of construction described in
later chapters has been quite different from that proposed.
SUMMARY
6.46. The procurement problem which had been delaying
progress was essentially solved by the end of 1942. A small self-
Metallurgical Project in 1942 107
sustaining graphite-uranium pile was constructed in November
1942, and was put into operation for the first time on December 2,
1942, at a power level of }/ watt and later at 200 watts. It was
easily controllable thanks to the phenomenon of delayed neutron
emission. A total of 500 micrograms of plutonium was made with
the cyclotron and separated chemically from the uranium and
fission products. Enough was learned of the chemistry of plu-
tonium to indicate the possibility of separation on a relatively
large scale. No great advance was made on bomb theory, but
calculations were checked and experiments with fast neutrons
extended. If anything, the bomb prospects looked more favorable
than a year earlier.
6.47. Enough experimenting and planning were done to
delineate the problems to be encountered in constructing and
operating a large-scale production plant. Some progress was
made in choice of type of plant, first choice at that time being a
pile of metallic uranium and graphite, cooled either by helium
or water. A specific program was drawn up for the construction
of pilot and production plants. This program presented time and
cost estimates.
CHAPTER VII. THE PLUTONIUM PRODUCTION
PROBLEM AS OF FEBRUARY 1943
INTRODUCTION
NEED OF DECISIONS
7.1. By the first of January 1943, the Metallurgical Laboratory
had achieved its first objective, a chain-reacting pile, and was
well on the way to the second, a process for extracting the
plutonium produced in such a pile. It was clearly time to formu-
late more definite plans for a production plant. The policy de-
cisions were made by the Policy Committee (see Chapter V) on
the recommendations from the Laboratory Director (A. H.
Compton), from the S-l Executive Committee, and from the
Reviewing Committee that had visited Chicago in December
1942. The only decisions that had already been made were that
the first chain-reacting pile should be dismantled and then
reconstructed on a site a short distance from Chicago and that
a 1,000-kilowatt plutonium plant should be built at Clinton,
Tennessee.
THE SCALE OF PRODUCTION
7.2. The first decision to be made was on the scale of pro-
duction that should be attempted. For reasons of security the
figure decided upon may not be disclosed here. It was very large.
THE MAGNITUDE OF THE PROBLEM
7.3. As we have seen, the production of one gram of plutonium
per day corresponds to a generation of energy at the rate of
500 to 1,500 kilowatts. Therefore a plant for large-scale produc-
tion of plutonium will release a very large amount of energy.
108
Plutonium Problem February 1943 109
The problem therefore was to design a plant of this capacity on
the basis of experience with a pile that could operate at a power
level of only 0.2 kilowatt. As regards the plutonium separation
work, which was equally important, it was necessary to draw
plans for an extraction and purification plant which would
separate some grams a day of plutonium from some tons of
uranium, and such planning had to be based on information
obtained by microchemical studies involving only half a milli-
gram of plutonium. To be sure, there w*as information available
for the design of the large-scale pile and separation plant from
auxiliary experiments and from large-scale studies of separation
processes using uranium as a stand-in for plutonium, but even
so the proposed extrapolations both as to chain-reacting piles
and as to separation processes were staggering. In peacetime no
engineer or scientist in his right mind would consider making
such a magnification in a single stage, and even in wartime only
the possibility of achieving tremendously important results
could justify it.
ASSIGNMENT OF RESPONSIBILITY
7.4. As soon as it had been decided to go ahead with large-
scale production of plutonium, it was evident that a great ex-
pansion in organization was necessary. The Stone and Webster
Engineering Corporation had been selected as the overall engi-
neering and construction firm for the DSM Project soon after
the Manhattan District was placed in charge of construction
work in June 1942. By October 1942, it became evident that
various component parts of the work were too far separated
physically and were too complicated technically to be handled
by a single company especially in view of the rapid pace re-
quired. Therefore it was decided that it would be advantageous
if Stone and Webster were relieved of that portion of the work
pertaining to the construction of plutonium production facilities.
This was done, and General Groves selected the E. I. du Pont
de Nemours and Company as the firm best able to carry on this
phase of the work. The arrangements made with various indus-
110 Plutonium Problem February 1943
trial companies by the Manhattan District took various forms.
The arrangements with du Pont are discussed in detail as an
example.
7.5. General Groves broached the question to W. S. Carpenter,
Jr., president of du Pont, and after considerable discussion with
him and other officials of the firm, du Pont agreed to undertake
the work. In their acceptance, they made it plain and it was
understood by all concerned that du Pont was undertaking the
work only because the War Department considered the work to
be of the utmost importance, and because General Groves stated
that this view as to importance was one held personally by the
President of the United States, the Secretary of War, the Chief
of Staff, and General Groves, and because of General Groves'
assertion that du Pont was by far the organization best qualified
for the job. At the same time, it was recognized that the du Pont
Company already had assumed all the war-connected activities
which their existing organization could be expected to handle
without undue difficulty.
7.6. The du Pont Company, in accepting the undertaking,
insisted that the work be conducted without profit and without
patent rights of any kind accruing to them. The du Pont Com-
pany did request, however, that in view of the unknown character
of the field into which they were being asked to embark, and in
view of the unpredictable hazards involved, the Government
provide maximum protection against losses sustained by du Pont.
7.7. The cost-plus-a-fixed-fee contract between the Govern-
ment and du Pont established a fixed fee of SI. 00. The Govern-
ment agreed to pay all costs of the work by direct reimbursement
or through allowances provided by the contract to cover ad-
ministrative and general expenses allocated to the work in accord-
ance with normal du Pont accounting practices as determined by
audit by certified public accountants. Under the terms of the
contract, any portion of these allowances not actually expended
by du Pont will, at the conclusion of the work, be returned to the
United States. The contract also provided that no patent rights
would accrue to the company.
Plutonium Problem February 1943 111
7.8. The specific responsibilities assumed by du Pont were to
engineer, design, and construct a small-scale semi- works at the
Clinton Engineer Works in Tennessee and to engineer, design,
construct, and operate a large-scale plutonium production plant
of large capacity at the Hanford Engineer Works in the State
of Washington. Because of its close connection with fundamental
research, the Clinton semi-works was to be operated under the
direction of the University of Chicago. A large number of key
technical people from du Pont were to be used on a loan basis at
Chicago and at Clinton, to provide the University with much
needed personnel, particularly men with industrial experience,
and to train certain of such personnel for future service at Hanford.
7.9. Inasmuch as du Pont was being asked to step out of its
normal role in chemistry into a new field involving nuclear
physics, it was agreed that it would be necessary for them to
depend most heavily upon the Metallurgical Laboratory of the
University of Chicago for fundamental research and development
data and for advice. The du Pont Company had engineering and
industrial experience, but it needed the Metallurgical Laboratory
for nuclear-physics and radiochemistry experience. The Metal-
lurgical Laboratory conducted the fundamental research on
problems bearing on the design and operation of the semi-works
and large-scale production plants. It proposed the essential parts
of the plutonium production and recovery processes and equip-
ment, answered the many specific questions raised by du Pont,
and studied and concurred in the final du Pont decisions and
designs.
7.10. The principal purpose of the Clinton semi- works was
development of methods of operation for plutonium recovery.
The semi-works had to include of course, a unit for plutonium
production, in order to provide plutonium to be recovered experi-
mentally. In the time and with the information available, the
Clinton production unit could not be designed to be an early
edition of the Hanford production units which, therefore, had to
be designed, constructed and operated without major guidance
from Clinton experience. In fact, even the Hanford recover}'
112 Plutonium Problem February 1943
units had to be far along in design and procurement of equip-
ment before Clinton results became available. However, the
Clinton semi-works proved to be an extremely important tool in
the solution of the many completely new problems encountered
at Hanford. It also produced small quantities of plutonium
which, along with Metallurgical Laboratory data on the prop-
erties of plutonium, enabled research in the use of this material
to be advanced many months.
CHOICE OF PLANT SITE
7.11. Once the scale of production had been agreed upon
and the responsibilities assigned, the nature of the plant and its
whereabouts had to be decided. The site in the Tennessee Valley,
known officially as the Clinton Engineer Works, had been ac-
quired by the Army for the whole program as recommended in
the report to the President (see Chapter V).
7.12. Reconsideration at the end of 1942 led General Groves
to the conclusion that this site was not sufficiently isolated for a
large-scale plutonium production plant. At that time, it was
conceivable that conditions might arise under which a large pile
might spread radioactive material over a large enough area to
endanger neighboring centers of population. In addition to the
requirement of isolation, there remained the requirement of a
large power supply which had originally determined the choice
of the Tennessee site. To meet these two requirements a new site
was chosen and acquired on the Columbia River in the central
part of the State of Washington near the Grand Coulee power
line. This site was known as the Hanford Engineer Works.
7.13. Since the Columbia River is the finest supply of pure
cold river water in this country, the Hanford site was well suited
to either the helium-cooled plant originally planned or to the
water-cooled plant actually erected. The great distances sepa-
rating the home office of du Pont in Wilmington, Delaware, the
pilot plant at Clinton, Tennessee, the Metallurgical Laboratory
at Chicago, and the Hanford site were extremely inconvenient,
but this separation could not be avoided. Difficulties also were
Plutonium Problem February 1943 113
inherent in bringing workmen to the site and in providing living
accommodations for them.
CHOICE OF TYPE OF PLANT
7.14. It was really too early in the development to make a
carefully weighed decision as to the best type of plutonium pro-
duction plant. Yet a choice had to be made so that design could
be started and construction begun as soon as possible. Actually
a tentative choice was made and then changed.
7.15. In November 1942, the helium-cooled plant was the
first choice of the Metallurgical Laboratory. Under the direction
of T. Moore and M. C. Leverett, preliminary plans for such
a plant had been worked out. The associated design studies were
used as bases for choice of site, choice of accessory equipment,
etc. Although these studies had been undertaken partly because
it had been felt that they could be carried through more quickly
for a helium-cooled plant than for a water-cooled plant, many
difficulties were recognized. Meanwhile the theoretical group
under Wigner, with the cooperation of the engineering personnel,
had been asked to prepare a report on a water-cooled plant of
high power output. This group had been interested in water-
cooling almost from the beginning of the project and was able to
incorporate the results of its studies in a report issued on January
9, 1943. This report contained many important ideas that were
incorporated in the design of the production plant erected at
Hanford.
7.16. When du Pont came into the picture, it at first accepted
the proposal of a helium-cooled plant but after further study
decided in favor of water cooling. The reasons for the change
were numerous. Those most often mentioned were the hazard
from leakage of a high-pressure gas coolant carrying radioactive
impurities, the difficulty of getting large blowers quickly, the
large amount of helium required, the difficulty of loading and
unloading uranium from the pile, and the relatively low power
output per kilogram of uranium metal. These considerations
had to be balanced against the peculiar disadvantages of a water-
114 Plutonium Problem February 1943
cooled plant, principally the greater complexity of the pile itself
and the dangers of corrosion.
7.17. Like so many decisions in this project, the choice between
various types of plant had to be based on incomplete scientific
information. The information is still incomplete, but there is
general agreement that water cooling was the wise choice.
THE PROBLEMS OF PLANT DESIGN
SPECIFICATION OF THE OVERALL PROBLEM
7.18. In Chapter II of this report we attempted to define
the general problem of the uranium project as it appeared in the
summer of 1940. We now wish to give precise definition to the
problem of the design of a large-scale plant for the production
of plutonium. The objective had already been delimited by
decisions as to scale of production, type of plant, and site. As it
then stood, the specific problem was to design a water-cooled
graphite-moderated pile (or several such piles) with associated
chemical separation plant to produce a specified, relatively large
amount of plutonium each day, the plant to be built at the Han-
ford site beside the Columbia River. Needless to say, speed of
construction and efficiency of operation were prime considerations.
NATURE OF THE LATTICE
7.19. The lattices we have been describing heretofore con-
sisted of lumps of uranium imbedded in the graphite moderator.
There are two objections to such a type of lattice for production
purposes: first, it is difficult to remove the uranium without dis-
assembling the pile; second, it is difficult to concentrate the
coolant at the uranium lumps, which are the points of maximum
production of heat. It was fairly obvious that both these diffi-
culties could be avoided if a rod lattice rather than a point lattice
could be used, that is, if the uranium could be concentrated
along lines passing through the moderator instead of being situated
merely at points. There was little doubt that the rod arrange-
ment would be excellent structurally and mechanically, but
Plutonium Problem February 1943 115
there was real doubt as to whether it was possible to build such
a lattice which would still have a multiplication factor k greater
than unity. This became a problem for both the theoretical and
experimental physicists. The theoretical physicists had to compute
what was the optimum spacing and diameter of uranium rods;
the experimental physicists had to perform exponential experi-
ments on lattices of this type in order to check the findings of
the theoretical group.
LOADING AND UNLOADING
7.20. Once the idea of a lattice with cylindrical symmetry
was accepted, it became evident that the pile could be unloaded
and reloaded without disassembly since the uranium could be
pushed out of the cylindrical channels in the graphite moderator
and new uranium inserted. The decision had to be made as to
whether the uranium should be in the form of long rods, which
had advantages from the nuclear-physics point of view, or of
relatively short cylindrical pieces, which had advantages from
the point of view of handling. In either case, the materials would
be so very highly radioactive that unloading would have to be
carried out by remote control, and the unloaded uranium would
have to be handled by remote control from behind shielding.
POSSIBLE MATERIALS; CORROSION
7.21. If water was to be used as coolant, it would have to be
conveyed to the regions where heat was generated through
channels of some sort. Since graphite pipes were not practical,
some other kind of pipe would have to be used. But the choice
of the material for the pipe, like the choice of all the materials
to be used hi the pile, was limited by nuclear-physics considera-
tions. The pipes must be made of some material whose absorption
cross section for neutrons was not large enough to bring the value
of k below unity. Furthermore, the pipes must be made of mate-
rial which would not disintegrate under the heavy density of
neutron and gamma radiation present in the pile. Finally, the
pipes must meet all ordinary requirements of cooling-system
116 Plutonium Problem February 1943
pipes: they must not leak; they must not corrode; they must not
warp.
7.22. From the nuclear-physics point of view there were seven
possible materials (Pb, Bi, Be, Al, Mg, Zn, Sn), none of which
had high neutron-absorption cross sections. No beryllium tubing
was available, and of all the other metals only aluminum was
thought to be possible from a corrosion point of view. But it
was by no means certain that aluminum would be satisfactory,
and doubts about the corrosion of the aluminum pipe were not
settled until the plant had actually operated for some time.
7.23. While the choice of material for the piping was very
difficult, similar choices involving both nuclear-physics criteria
and radiation-resistance criteria had to be made for all other
materials that were to be used in the pile. For example, the elec-
tric insulating materials to be used in any instruments buried
in the pile must not disintegrate under the radiation. In certain
instances where control or experimental probes had to be in-
serted and removed from the pile, the likelihood had to be borne
in mind that the probes would become intensely radioactive as
a result of their exposure in the pile and that the degree to which
this would occur would depend on the material used.
7.24. Finally, it was not known what effect the radiation fields
in the pile would have on the graphite and the uranium. It
was later found that the electric resistance, the elasticity, and the
heat conductivity of the graphite all change with exposure to
intense neutron radiation.
PROTECTION OF THE URANIUM FROM CORROSION
7.25. The most efficient cooling procedure would have been
to have the water flowing in direct contact with the uranium in
which the heat was being produced. Indications were that this
was probably out of the question because the uranium would
react chemically with the water, at least to a sufficient extent to
put a dangerous amount of radioactive material into solution
and probably to the point of disintegrating the uranium slugs.
Therefore it was necessary to find some method of protecting the
Plutonium Problem February 1943 117
uranium from direct contact with the water. Two possibilities
were considered: one was some sort of coating, either by electro-
plating or dipping; the other was sealing the uranium slug in a
protective jacket or "can." Strangely enough, this "canning
problem" turned out to be one of the most difficult problems
encountered in such piles.
WATER SUPPLY
7.26. The problem of dissipating thousands of kilowatts of
energy is by no means a small one. How much water was needed
depended, of course, on the maximum temperature to which the
water could safely be heated and the maximum temperature to
be expected in the intake from the Columbia River; certainly
the water supply requirement was comparable to that of a fair-
sized city. Pumping stations, filtration and treatment plants all
had to be provided. Furthermore, the system had to be a very
reliable one; it was necessary to provide fast-operating controls
to shut down the chain-reacting unit in a hurry in case of failure
of the water supply. If it was decided to use "once-through"
cooling instead of recirculation, a retention basin would be
required so that the radioactivity induced in the water might
die down before the water was returned to the river. The volume
of water discharged was going to be so great that such problems
of radioactivity were important, and therefore the minimum
time that the water must be held for absolute safety had to be
determined.
CONTROLS AND INSTRUMENTATION
7.27. The control problem was very similar to that discussed in
connection with the first chain-reacting pile except that every-
thing was on a larger scale and was, therefore, potentially more
dangerous. It was necessary to provide operating controls which
would automatically keep the pile operating at a determined
power level. Such controls had to be connected with instruments
in the pile which would measure neutron density or some other
property which indicated the power level. There would also have
118 Plutonium Problem February 1943
to be emergency controls which would operate almost instan-
taneously if the power level showed signs of rapid increase or if
there was any interruption of the water supply. It was highly
desirable that there be some means of detecting incipient diffi-
culties such as the plugging of a single water tube or a break in
the coating of one of the uranium slugs. All these controls and
instruments had to be operated from behind the thick shielding
walls described below.
SHIELDING
7.28. As we have mentioned a number of times, the radiation
given off from a pile operating at a high power level is so strong
as to make it quite impossible for any of the operating personnel
to go near the pile. Furthermore, this radiation, particularly the
neutrons, has a pronounced capacity for leaking out through
holes or cracks in barriers. The whole of a power pile therefore
has to be enclosed in very thick walls of concrete, steel, or other
absorbing material. But at the same time it has to be possible to
load and unload the pile through these shields and to carry the
water supply in and out through the shields. The shields should
not only be radiation-tight but air-tight since air exposed to the
radiation in the pile would become radioactive.
7.29. The radiation dangers that require shielding in the pile
continue through a large part of the separation plant. Since the
fission products associated with the production of the plutonium
are highly radioactive, the uranium after ejection from the pile
must be handled by remote control from behind shielding and
must be shielded during transportation to the separation plant.
All the stages of the separation plant, including analyses, must
be handled by remote control from behind shields up to the
point where the plutonium is relatively free of radioactive fission
products.
MAINTENANCE
7.30. The problem of maintenance is very simply stated. There
could not be any maintenance inside the shield or pile once the
Plutonium Problem February 1943 119
pile had operated. The same remark applies to a somewhat lesser
extent to the separation unit, where it was probable that a shut-
down for servicing could be effected, provided, of course, that
adequate remotely-controlled decontamination processes were
carried out in order to reduce the radiation intensity below the
level dangerous to personnel. The maintenance problem for the
auxiliary parts of the plant was normal except for the extreme
importance of having stand-by pumping and power equipment
to prevent a sudden accidental breakdown of the cooling system.
SCHEDULE OF LOADING AND UNLOADING
7.31. Evidently the amount of plutonium in an undisturbed
operating pile increases with time of operation. Since Pu-239
itself undergoes fission its formation tends to maintain the chain
reaction, while the gradual disappearance of the U-235 and the
appearance of fission products with large neutron absorption
cross sections tend to stop the reaction. The determination of
when a producing pile should be shut down and the plutonium
extracted involves a nice balancing of these factors against tune
schedules, material costs, separation-process efficiency, etc.
Strictly speaking, this problem is one of operation rather than
of design of the plant, but some thought had to be given to it in
order to plan the flow of uranium slugs to the pile and from the
pile to the separation plant.
SIZE OF UNITS
7.32. We have been speaking of the production capacity of the
plant only in terms of overall production rate. Naturally, a given
rate of production might be achieved in a single large pile or in
a number of smaller ones. The principal advantage of the smaller
piles would be the reduction in construction time for the first
pile, the possibility of making alterations hi later piles, and
perhaps most important the improbability of simultaneous
breakdown of all piles. The disadvantage of small piles is that
they require disproportionately large amounts of uranium,
120 Plutonium Problem February 1943
moderator, etc. There is, in fact, a preferred "natural size" of
pile which can be roughly determined on theoretical grounds.
GENERAL NATURE OF THE SEPARATION PLANT
7.33. As we have already pointed out, the slugs coming from
the pile are highly radioactive and therefore must be processed
by remote control in shielded compartments. The general scheme
to be followed was suggested in the latter part of 1 942, particularly
in connection with plans for the Clinton separation plant. This
scheme was to build a "canyon" which would consist of a series
of compartments with heavy concrete walls arranged in a line
and almost completely buried in the ground. Each compartment
would contain the necessary dissolving or precipitating tanks or
centrifuges. The slugs would come into the compartment at one
end of the canyon; they would then be dissolved and go through
the various stages of solution, precipitation, oxidation, or reduc-
tion, being pumped from one compartment to the next until a
solution of plutonium free from uranium and fission products
came out in the last compartment. As in the case of the pile,
everything would be operated by remote control from above
ground, but the operations would be far more complicated than
in the case of the pile. However, as far as the chemical operations
themselves were concerned, their general nature was not so far
removed from the normal fields of activity of the chemists
involved.
ANALYTICAL CONTROL
7.34. In the first stages of the separation process even the
routine analysis of samples which was necessary in checking the
operation of the various chemical processes had to be done by
remote control. Such testing was facilitated, however, by use of
radioactive methods of analysis as well as conventional chemical
analyses.
WASTE DISPOSAL
7.35. The raw material (uranium) is not dangerously radio-
active. The desired product (plutonium) does not give off pene-
Plutonium Problem February 1943 121
trating radiation, but the combination of its alpha-ray activity
and chemical properties makes it one of the most dangerous sub-
stances known if it once gets into the body. However, the really
troublesome materials are the fission products, i.e., the major
fragments into which uranium is split by fission. The fission
products are very radioactive and include some thirty elements.
Among them are radioactive xenon and radioactive iodine.
These are released in considerable quantity when the slugs are
dissolved and must be disposed of with special care. High stacks
must be built which will carry off these gases along with the acid
fumes from the first dissolving unit, and it must be established
that the mixing of the radioactive gases with the atmosphere will
not endanger the surrounding territory. (As in all other matters
of health, the tolerance standards that were set and met were so
rigid as to leave not the slightest probability of danger to the
health of the community or operating personnel.)
7.36. Most of the other fission products can be retained in
solution but must eventually be disposed of. Of course, possible
pollution of the adjacent river must be considered. (In fact, the
standards of safety set and met with regard to river pollution
were so strict that neither people nor fish down the river can
possibly be affected.)
RECOVERY OF URANIUM
7.37. Evidently, even if the uranium were left in the pile until
all the U-235 had undergone fission, there would still be a large
amount of U-238 which had not been converted to plutonium.
Actually the process is stopped long before this stage is reached.
Uranium is an expensive material and the total available supply
is seriously limited. Therefore the possibility of recovering it after
the plutonium is separated must be considered. Originally there
was no plan for recovery, but merely the intention of storing
the uranium solution. Later, methods of large-scale recovery
were developed
122 Plutonium Problem February 1943
CORROSION IN THE SEPARATION PLANT
7.38. An unusual feature of the chemical processes involved
was that these processes occur in the presence of a high density
of radiation. Therefore the containers used may corrode more
rapidly than they would under normal circumstances. Further-
more, any such corrosion will be serious because of the difficulty
of access. For a long time, information was sadly lacking on these
dangers.
EFFECT OF RADIATION ON CHEMICAL REACTIONS
7.39. The chemical reactions proposed for an extraction
process were, of course, tested in the laboratory. However, they
could not be tested with appreciable amounts of plutonium nor
could they be tested in the presence of radiation of anything like
the expected intensity. Therefore it was realized that a process
found to be successful in the laboratory might not work in the
plant.
CHOICE OF PROCESS
7.40. The description given above as to what was to happen
in the successive chambers in the canyon was very vague. This
was necessarily so, since even by January 1943 no decision had
been made as to what process would be used for the extraction
and purification of plutonium. The major problem before the
Chemistry Division of the Metallurgical Laboratory was the
selection of the best process for the plant.
THE HEALTH PROBLEM
7.41. Besides the hazards normally present during construction
and operation of a large chemical plant, dangers of a new kind
were expected here. Two types of radiation hazard were antici-
pated neutrons generated in the pile, and alpha particles,
beta particles, and gamma rays emitted by products of the pile.
Although the general effects of these radiations had been proved
Plutonium Problem February 1943 123
to be similar to those of X-rays, very little detailed knowledge
was available. Obviously the amounts of radioactive material
to be handled were many times greater than had ever been en-
countered before.
7.42. The health group had to plan three programs: (1) pro-
vision of instruments and clinical tests to detect any evidence of
dangerous exposure of the personnel; (2) research on the effects
of radiation on persons, instruments, etc.; and (3) estimates of
what shielding and safety measures must be incorporated in the
design and plan of operation of the plant.
THE PROPERTIES OF PLUTONIUM
7.43. Although we were embarking on a major enterprise to
produce plutonium, we still had less than a milligram to study
and still had only limited familiarity with its properties. The
study of plutonium, therefore, remained a major problem of the
Metallurgical Laboratory.
THE TRAINING OF OPERATORS
7.44. Evidently the operation of a full-scale plant of the type
planned would require a large and highly skilled group of oper-
ators. Although du Pont had a tremendous background of
experience in the operation of various kinds of chemical plant,
this was something new and it was evident that operating per-
sonnel would need special training. Such training was carried
out partly in Chicago and its environs, but principally at the
Clinton Laboratories.
THE NEED FOR FURTHER INFORMATION
7.45. In the preceding paragraphs of this chapter we have
outlined the problems confronting the group charged with
designing and building a plutonium production plant. In Chapter
VI the progress in this field up to the end of 1942 was reviewed.
Throughout these chapters it is made clear that a great deal more
124 Plutonium Problem February 1943
information was required to assure the success of the plant.
Such answers as had been obtained to most of the questions were
only tentative. Consequently research had to be pushed simul-
taneously with planning and construction.
THE RESEARCH PROGRAM
7.46. To meet the need for further information, research pro-
grams were laid out for the Metallurgical Laboratory and the
Clinton Laboratory. The following passage is an excerpt from
the 1943 program of the Metallurgical Project:
"Product Production Studies. These include all aspects of the re-
search, development and semi-works studies necessary for the
design, construction, and operation of chain-reacting piles to
produce plutonium or other materials.
Pile Characteristics. Theoretical studies and experiments on
lattice structures to predict behavior in high-level piles, such
as temperature and barometric effects, neutron character-
istics, pile poisoning, etc.
Control of Reacting Units. Design and experimental tests of
devices for controlling rate of reaction in piles.
Cooling of Reacting Units. Physical studies of coolant material,
engineering problems of circulation, corrosion, erosion, etc.
Instrumentation. Development of instruments and technique
for monitoring pile and surveying radiation throughout
plant area.
Protection. Shielding, biological effects of radiation at pile
and clinical effects of operations associated with pile.
Materials. Study of physical (mechanical and nuclear) prop-
erties of construction and process materials used in pile
construction and operation.
Activation Investigations. Production of experimental amounts
of radioactive materials in cyclotron and in piles and study
of activation of materials by neutrons, protons, electrons,
gamma rays, etc.
Plutonium Problem February 1943 125
Pile Operation. Study of pile operation procedures such as
materials handling, instrument operation, etc.
Process Design. Study of possible production processes as a
whole leading to detailed work in other categories.
"Product Recovery Studies. These include all aspects of the work
necessary for the development of processes for the extraction of
plutonium and possible by-products from the pile material and
their preparation in purified form. Major effort at the Metal-
lurgical Laboratory will be on a single process to be selected by
June 1, 1943 for the production of plutonium, but alternatives
will continue to be studied both at the Metallurgical Laboratory
and Clinton with whatever manpower is available.
Separation. Processes for solution of uranium, extraction of
plutonium and decontamination by removal of fission
products.
Concentration, Purification and Product Reduction. Processes lead-
ing to production of plutonium as pure metal, and study of
properties of plutonium necessary to its production.
Wastes. Disposal and possible methods of recovery of fission
products and metal from wastes.
Instrumentation. Development and testing of instruments for
monitoring chemical processes and surveying radiation
throughout the area.
Protection. Shi-elding studies, determination of biological
effects of radioactive dusts, liquids, solids, and other process
materials, and protective measures.
Materials. Corrosion of equipment materials, and radiation
stability. Necessary purity and purity analysis of process
materials, etc.
Recovery of Activated Materials. Development of methods and
actual recovery of activated material (tracers, etc.) from
cyclotron and pile-activated materials.
126 Plutonium Problem February 1943
Operations Studies. Equipment performance, process control,
material handling operations, etc.
Process Design. Study of product recovery processes as a
whole (wet processes, physical methods) leading to detailed
work in other categories.
"Fundamental Research. Studies of the fundamental physical,
chemical and biological phenomena occurring in chain-reacting
piles, and basic properties of all materials involved. Although
the primary emphasis at Clinton is on the semi-works level,
much fundamental research will require Clinton conditions (high
radiation intensity, large scale processes).
Nuclear Physics. Fundamental properties of nuclear fission
such as cross section, neutron yield, fission species, etc.
Other nuclear properties important to processes, such as
cross sections, properties of moderators, neutron effect on
materials, etc.
General Physics. Basic instrument (electronic, ionization,
optical, etc.) research, atomic mass determinations, neutron,
a, /3, 7 radiation studies, X-ray investigations, etc.
Radiation Chemistry. Effects of radiation on chemical proc-
esses and chemical reactions produced by radiation.
Nuclear Chemistry. Tracing of fission products, disintegration
constants, chains, investigation of nuclei of possible use to
project.
Product Chemistry. Chemical properties of various products
and basic studies in separation and purification of products.
General Chemistry. Chemistry of primary materials and mate-
rials associated with process, including by-products.
General Biology. Fundamental studies of effects of radiation
on living matter, metabolism of important materials, etc.
Clinical Investigations. Basic investigations, such as hema-
tology, pathology, etc.
Metallurgical Studies. Properties of U, Pu, Be, etc.
Plutonium Problem February 1943 127
Engineering Studies. Phenomena basic to corrosion and similar
studies essential to continued engineering development of
processes."
7.47. An examination of this program gives an idea of the
great range of investigations which were considered likely to
give relevant information. Many of the topics listed are not
specific research problems such as might be solved by a small
team of scientists working for a few months but are whole fields
of investigation that might be studied with profit for years. It was
necessary to pick the specific problems that were likely to give
the most immediately useful results but at the same time it was
desirable to try to uncover general principles. For example, the
effect of radiation on the properties of materials ("radiation
stability") was almost entirely unknown. It was necessary both
to make empirical tests on particular materials that might be
used in a pile and to devise general theories of the observed
effects. Every effort was made to relate all work to the general
objective: a successful production plant.
ORGANIZATION OF THE PROJECT
7.48. There have been many changes in the organization and
personnel of the project. During most of the period of construc-
tion at Clinton and Hanford, A. H. Compton was director of the
Metallurgical Project; S. K. Allison was director of the Metal-
lurgical Laboratory at Chicago; and M. D. Whitaker was direc-
tor of the Clinton Laboratory. The Chicago group was organized
in four divisions: physics, chemistry, technology, and health.
Later the Physics Division was split into general physics and
nuclear physics. R. L. Doan was research director at Clinton
but there was no corresponding position at Chicago. Among
others who have been associate or assistant laboratory or project
directors or have been division directors are S. T. Cantril, C. M.
Cooper, F. Daniels, A. J. Dempster, E. Fermi, J. Franck,
N. Hilberry, T. R. Hogness, W. C. Johnson, H. D. Smyth, J. C.
Stearns, R. S. Stone, H. C. Vernon, W. W. Watson, and E.
128 Plutonium Problem February 1943
Wigner. Beginning in 1943 C. H. Thomas of the Monsanto
Chemical Company acted as chairman of a committee on the
Chemistry and Metallurgy of Plutonium. This committee cor-
related the activities of the Metallurgical Laboratory with those
at Los Alamos (see Chapter XII) and elsewhere. Later the
Monsanto Chemical Company did some work on important
special problems arising in connection with the Los Alamos work.
7.49. It was the responsibility of these men to see that the
research program described above was carried out and that
significant results were reported to du Pont. It was their responsi-
bility also to answer questions raised by du Pont and to approve
or criticize plans submitted by du Pont.
COOPERATION BETWEEN THE METALLURGICAL
LABORATORY AND DU PONT
7.50. Since du Pont was the design and construction organiza-
tion and the Metallurgical Laboratory was the research organiza-
tion, it was obvious that close cooperation was essential. Not only
did du Pont need answers to specific questions, but they could
benefit by criticism and suggestions on the many points where
the Metallurgical group was especially well-informed. Similarly,
the Metallurgical group could profit by the knowledge of du Pont
on many technical questions of design, construction, and opera-
tion. To promote this kind of cooperation du Pont stationed one
of their physicists, J. B. Miles, at Chicago, and had many other
du Pont men, particularly C. H. Greenewalt, spend much of
their time at Chicago. Miles and Greenewalt regularly attended
meetings of the Laboratory Council. There was no similar
reciprocal arrangement although many members of the labora-
tory visited Wilmington informally. In addition, J. A. Wheeler
was transferred from Chicago to Wilmington and became a
member .of the du Pont staff. There was, of course, constant
exchange of reports and letters, and conferences were held fre-
quently between Compton and R. Williams of du Pont. Whitaker
spent much of his time at Wilmington during the period when the
Clinton plant was being designed and constructed.
Plutonium Problem February 1943 129
SUMMARY
7.51. By January 1943, the decision had been made to build
a plutonium production plant with a large capacity. This meant
a pile developing thousands of kilowatts and a chemical separa-
tion plant to extract the product. The du Pont Company was to
design, construct, and operate the plant; the Metallurgical Labor-
atory was to do the necessary research. A site was chosen on the
Columbia River at Hanford, Washington. A tentative decision
to build a helium-cooled plant was reversed in favor of water-
cooling. The principal problems were those involving lattice
design, loading and unloading, choice of materials particularly
with reference to corrosion and radiation, water supply, controls
and instrumentation, health hazards, chemical separation process,
and design of the separation plant. Plans were made for the neces-
sary fundamental and technical research and for the training of
operators. Arrangements were made for liaison between du Pont
and the Metallurgical Laboratory.
CHAPTER VIII. THE PLUTONIUM PROBLEM
JANUARY 1943 TO JUNE 1945
INTRODUCTION
8.1. The necessity for pushing the design and construction of
the full-scale plutonium plant simultaneously with research and
development inevitably led to a certain amount of confusion
and inefficiency. It became essential to investigate many alter-
native processes. It became necessary to investigate all possible
causes of failure even when the probability of their becoming
serious was very small. Now that the Hanford plant is producing
plutonium successfully, we believe it is fair to say that a large
percentage of the results of investigation made between the end
of 1942 and the end of 1944 will never be used at least not for
the originally intended purposes. Nevertheless had the Hanford
plant run into difficulties, any one of the now superfluous in-
vestigations might have furnished just the information required
to convert failure into success. Even now it is impossible to say
that future improvements may not depend on the results of
researches that seem unimportant today.
8.2. It is estimated that thirty volumes will be required for a
complete report of the significant scientific results of researches
conducted under the auspices of the Metallurgical Project.
Work was done on every item mentioned on the research program
presented in the last chapter. In the present account it would be
obviously impossible to give more than a brief abstract of all
these researches. We believe this would be unsatisfactory and
that it is preferable to give a general discussion of the chain-
reacting units and separation plants as they now operate, with
some discussion of the earlier developments.
THE CHAIN REACTION IN A PILE
8.3. In Chapter I and other early chapters we have given
brief accounts of the fission process, pile operation, and chemical
130
Plutonium Problem to June 1945 131
separation. We shall now review these topics from a somewhat
different point of view before describing the plutonium produc-
tion plants themselves.
8.4. The operation of a pile depends on the passage of neutrons
through matter and on the nature of the collisions of neutrons
with the nuclei encountered. The collisions of principal impor-
tance are the following:
I. Collisions in which neutrons are scattered and lose appreci-
able amounts of energy, (a) Inelastic collisions of fast neutrons
with uranium nuclei, (b) Elastic collisions of fast or moderately
fast neutrons with the light nuclei of the moderator material;
these collisions serve to reduce the neutron energy to very low
(so-called thermal) energies.
II. Collisions in which the neutrons are absorbed, (a) Colli-
sions which result in fission of nuclei and give fission products and
additional neutrons, (b) Collisions which result in the formation
of new nuclei which subsequently disintegrate radioactively
(e.g., wU 239 which produces 94 Pu 239 ).
8.5. Only the second class of collision requires further dis-
cussion. As regards collisions of Type II (a), the most important
in a pile are the collisions between neutrons and U-235, but the
high-energy fission of U-238 and the thermal fission of Pu-239
also take place. Collisions of Type II (b) are chiefly those between
neutrons and U-238. Such collisions occur for neutrons of all
energies, but they are most likely to occur for neutrons whose
energies lie in the "resonance" region located somewhat above
thermal energies. The sequence of results of the Type II (b)
collision is represented as follows:
M U 238 + on 1 -> 92 U 239 + gamma rays
92 U 239 ^j 93 Np 239 -f _ ie
93N P 239 TTdl^ 94Fu239 + - ie + g amma ra ys
8.6. Any other non-fission absorption processes are important
chiefly because they waste neutrons; they occur in the moderator,
132 Plutonium Problem to June 1945
in U-235, in the coolant, in the impurities originally present, in
the fission products, and even in plutonium itself.
8.7. Since the object of the chain reaction is to generate
plutonium, we would like to absorb all excess neutrons in U-238,
leaving just enough neutrons to produce fission and thus to
maintain the chain reaction. Actually the tendency of the
neutrons to be absorbed by the dominant isotope U-238 is so
great compared to their tendency to produce fission in the 140-
times-rarer U-235 that the principal design effort had to be
directed toward favoring the fission (as by using a moderator,
a suitable lattice, materials of high purity, etc.,) in order to
maintain the chain reaction.
LIFE HISTORY OF ONE GENERATION OF NEUTRONS*
8.8. All the chain-reacting piles designed by the Metallurgical
Laboratory or with its cooperation consist of four categories of
material the uranium metal, the moderator, the coolant, and
the auxiliary materials such as water tubes, casings of uranium,
control strips or rods, impurities, etc. All the piles depend on
stray neutrons from spontaneous fission or cosmic rays to initiate
the reaction.
8.9. Suppose that the pile were to be started by simultaneous
release (in the uranium metal) of N high-energy neutrons.
Most of these neutrons originally have energies above the thresh-
old energy of fission of U-238. However, as the neutrons pass
back and forth in the metal and moderator, they suffer numerous
inelastic collisions with the uranium and numerous elastic colli-
sions with the moderator, and all these collisions serve to reduce
the energies below that threshold. Specifically, in a typical
graphite-moderated pile a neutron that has escaped from the
uranium into the graphite travels on the average about 2.5 cm
between collisions and makes on the average about 200 elastic
collisions before passing from the graphite back into the uranium.
Since at each such collision a neutron loses on the average about
one sixth of its energy, a one-Mev neutron is reduced to thermal
* See drawing facing p. 35.
Plutonium Problem to June 1945 133
energy (usually taken to be 0.025 electron volt) considerably
before completing a single transit through the graphite. There
are, of course, many neutrons that depart from this average
behavior, and there will be enough fissions produced by fast
neutrons to enhance slightly the number of neutrons present.
The enhancement may be taken into account by multiplying
the original number of neutrons N by a factor which is called
the fast-fission effect or the fast-multiplication factor.
8.10. As the average energy of the N neutrons present con-
tinues to fall, inelastic collision in the uranium becomes unimpor-
tant, the energy being reduced essentially only in the moderator.
However, the chance of non-fission absorption (resonance cap-
ture) in U-238 becomes significant as the intermediate or reson-
ance energy region is reached. Actually quite a number of
neutrons in this energy region will be absorbed regardless of choice
of lattice design. The effect of such capture may be expressed by
multiplying N by a factor p, (which is always less than one)
called the "resonance escape probability" which is the proba-
bility that a given neutron starting with energy above the reson-
ance region will reach thermal energies without absorption in
U-238. Thus from the original N high-energy neutrons we obtain
Nep neutrons of thermal energy.
8.11. Once a neutron has reached thermal energy the chance
that it will lose more energy by collision is no greater than the
chance that it will gain energy. Consequently the neutrons will
remain at this average energy until they are absorbed. In the
thermal-energy region the chance for absorption of the neutron
by the moderator, the coolant and the auxiliary materials is
greater than at higher energies. At any rate it is found that we
introduce little error into our calculations by assuming all such
unwanted absorption takes place in this energy region. We now
introduce a factor f, called the thermal utilization factor, which
is defined as the probability that a given thermal neutron will be
absorbed in the uranium. Thus from the original N fast neutrons
we have obtained Ntpf thermal neutrons which are absorbed by
uranium.
134 Plutonium Problem to June 1945
8.12. Although there are several ways in which the normal
mixture of uranium isotopes can absorb neutrons, the reader may
recall that we defined in a previous chapter a quantity 77, which
is the number of fission neutrons produced for each thermal
neutron absorbed in uranium regardless of the details of the
process. If, therefore, we multiply the number of thermal neu-
trons absorbed in uranium, Ncpf, by 77, we have the number of
new high speed neutrons generated by the original N high speed
neutrons in the course of their lives. If Nepfr; is greater than N,
we have a chain reaction and the number of neutrons is con-
tinually increasing. Evidently the product epf?; = k^, the multi-
plication factor already defined in Chapter IV.
8.13. Note that no mention has been made of neutrons
escaping from the pile. Such mention has been deliberately
avoided since the value of k^ as defined above applies to an
infinite lattice. From the known values of k^ and the fact that
these piles do operate, one finds that the percentage of neutrons
escaping cannot be very great. As we saw in Chapter II, the
escape of neutrons becomes relatively less important as the size
of the pile increases. If it is necessary to introduce in the pile a
large amount of auxiliary material such as cooling-system pipes,
it is necessary to build a somewhat larger pile to counteract the
increase in absorption.
8.14. To sum up, a pile operates by reducing high-energy
neutrons to thermal energies by the use of a moderator-lattice
arrangement, then allowing the thermal-energy neutrons to be
absorbed by uranium, causing fission which regenerates further
high-energy neutrons. The regeneration of neutrons is aided
slightly by the fast neutron effect; it is impeded by resonance
absorption during the process of energy reduction, by absorption
in graphite and other materials, and by neutron escape.
THE EFFECTS OF REACTION PRODUCTS ON THE
MULTIPLICATION FACTOR
8.15. Even at the high power level used in the Hanford piles,
only a few grams of U-238 and of U-235 are used up per day per
Plutonium Problem to June 1945 135
million grams of uranium present. Nevertheless the effects of
these changes are very important. As the U-235 is becoming
depleted, the concentration of plutonium is increasing. For-
tunately, plutonium itself is fissionable by thermal neutrons and
so tends to counterbalance the decrease of U-235 as far as main-
taining the chain reaction is concerned. However, other fission
products are being produced also. These consist typically of
unstable and relatively unfamiliar nuclei so that it was originally
impossible to predict how great an undesirable effect they would
have on the multiplication constant. Such deleterious effects are
called poisoning.
THE REACTION PRODUCTS AND THE
SEPARATION PROBLEM
8.16. There are two main parts of the plutonium production
process at Hanford : actual production in the pile, and separation
of the plutonium from the uranium slugs in which it is formed.
We turn now to a discussion of the second part, the separation
process.
8.17. The uranium slugs containing plutonium also contain
other elements resulting from the fission of U-235. When a U-235
nucleus undergoes fission, it emits one or more neutrons and
splits into two fragments of comparable size and of total mass
235 or less. Apparently fission into precisely equal masses rarely
occurs, the most abundant fragments being a fragment of mass
number between 134 and 144 and a fragment of mass number
between 100 and 90. Thus there are two groups of fission prod-
ucts: a heavy group with mass numbers extending approxi-
mately from 127 to 154, and a light group from approximately
1 15 to 83. These fission products are in the main unstable isotopes
of the thirty or so known elements in these general ranges of mass
number. Typically they decay by successive beta emissions
accompanied by gamma radiation finally to form known stable
nuclei. The half-lives of the various intermediate nuclei range
from fractions of a second to a year or more; several of the
important species have half-lives of the order of a month or so.
136 Plutonium Problem to June 1945
About twenty different elements are present in significant con-
centration. The most abundant of these comprises slightly less
than 10 per cent of the aggregate.
8.18. In addition to radioactive fission products, U-239 and
Np-239 (intermediate products in the formation of plutonium)
are present in the pile and are radioactive. The concentrations
of all these products begin to build up at the moment the pile
starts operating. Eventually the rate of radioactive decay equals
the rate of formation so that the concentrations become constant.
For example, the number of atoms of U-239 produced per second
is constant for a pile operating at a fixed power level. According
to the laws of radioactive disintegration, the number of U-239
atoms disappearing per second is proportional to the number of
such atoms present and is thus increasing during the first few
minutes or hours after the pile is put into operation. Consequently
there soon will be practically as many nuclei disintegrating each
second as are formed each second. Equilibrium concentrations
for other nuclei will be approached in similar manner, the
equilibrium concentration being proportional to the rate of
formation of the nucleus and to its half-life. Products which are
stable or of extremely long half-life (e.g., plutonium) will steadily
increase in concentration for a considerable time. When the pile
is stopped, the radioactivity of course continues, but at a con-
tinually diminishing absolute rate. Isotopes of very short half-
life may "drop out of sight" in a few minutes or hours; others of
longer half-life keep appreciably active for days or months.
Thus at any time the concentrations of the various products in a
recently stopped pile depend on what the power level was, on
how long the pile ran, and on how long it has been shut down.
Of course, the longer the pile has run, the larger is the concentra-
tion of plutonium and (unfortunately) the larger is the concentra-
tion of long-lived fission products. The longer the "cooling"
period, i.e., the period between removal of material from the
pile and chemical treatment, the lower is the radiation intensity
from the fission products. A compromise must be made between
such considerations as the desire for a long running and cooling
Plutonium Problem to June 1945 137
time on the one hand and the desire for early extraction of the
plutonium on the other hand.
8.19. Tables can be prepared showing the chemical concentra-
tions of plutonium and the various fission products as functions
of power level, length of operation, and length of cooling period.
The half-life of the U-239 is so short that its concentration be-
comes negligible soon after the pile shuts down. The neptunium
becomes converted fairly rapidly to plutonium. Of course, the
total weight of fission products, stable and unstable, remains
practically constant after the pile is stopped. For the Clinton and
Hanford operating conditions the maximum plutonium con-
centration attained is so small as to add materially to the diffi-
culty of chemical separation.
THE CHOICE OF A CHEMICAL SEPARATION PROCESS
8.20. The problem then is to make a chemical separation at
the daily rate of, say, several grams of plutonium from several
thousand grams of uranium contaminated with large amounts of
dangerously radioactive fission products comprising twenty
different elements. The problem is especially difficult as the
plutonium purity requirements are very high indeed.
8.21. Four types of method for chemical separation were
examined: volatility, absorption, solvent extraction, and pre-
cipitation. The work on absorption and solvent extraction
methods has been extensive and such methods may be increas-
ingly used in the main process or in waste recovery, but the Han-
ford Plant was designed for a precipitation process.
8.22. * The phenomena of co-precipitation, i.e., the precipita-
tion of small concentrations of one element along with a "carrier"
precipitate of some other element, had been commonly used in
radioactive chemistry, and was adopted for plutonium separation.
The early work on plutonium chemistry, confined as it was to
minute amounts of the element, made great use of precipitation
* Paragraphs 8.22-8.26 are quoted or paraphrased from a general report
of the Metallurgical Laboratory prepared in the spring of 1945.
138 Plutonium Problem to June 1945
reactions from which solubility properties could be deduced.
It was therefore natural that precipitation methods of separation
were the most advanced at the time when the plant design was
started. It was felt that, should the several steps in the separations
process have to be developed partly by the empirical approach,
there would be less risk in the scale-up of a precipitation process
than, for example, of one involving solid-phase reactions. In
addition, the precipitation processes then in mind could be
broken into a sequence of repeated operations (called cycles),
thereby limiting the number of different equipment pieces
requiring design and allowing considerable process change
without equipment change. Thus, while the basic plant design
was made with one method in mind, the final choice of a different
method led to no embarrassments.
8.23. Most of the precipitation processes which have received
serious consideration made use of an alternation between the
(IV) and (VI) oxidation states of plutonium. Such processes
involve a precipitation of plutonium (IV) with a certain com-
pound as a carrier, then dissolution of the precipitate, oxidation
of the plutonium to the (VI) state, and reprecipitation of the
carrier compound while the plutonium (VI) remains in solution.
Fission products which are not carried by these compounds
remain in solution when plutonium (IV) is precipitated. The
fission products which carry are removed from the plutonium
when it is in the (VI) state. Successive oxidation-reduction
cycles are carried out until the desired decontamination is
achieved. The process of elimination of the fission products is
called decontamination and the degree of elimination is tested
by measuring the change in radioactivity of the material.
COMBINATION PROCESSES
8.24. It is possible to combine or couple the various types of
process. Some advantages may be gained in this way since one
type of process may supplement another. For example, a process
which gives good decontamination might be combined advan-
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Site diagram of the Hanford Engineer Works near Pasco, Wash.
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Initial test of the atomic bomb in New Mexico on July 16, from a distance of 6 miles.
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Center: Multi-colored cloud from the explosion. Black areas were brighter than the
sun itself, according to observers. Below. A later stage of the development of the cloud.
Plutonium Problem to June 1945 139
tageously with one which, while inefficient for decontamination,
would be very efficient for separation from uranium.
8.25. At the time when it became necessary to decide on the
process to serve as the basis for the design of the Hanford plant
(June 1943), the choice, for reasons given above, was limited to
precipitation processes and clearly lay between two such processes.
However, the process as finally chosen actually represented a
combination of the two.
8.26. The success of the separation process at Hanford has
exceeded all expectations. The high yields and decontamination
factors and the relative ease of operation have amply demon-
strated the wisdom of its choice as a process. This choice was
based on a knowledge of plutonium chemistry which had been
gleaned from less than a milligram of plutonium. Further devel-
opments may make the present Hanford process obsolete, but
the principal goal, which was to have a workable and efficient
process for use as soon as the Hanford piles were delivering pluton-
ium, has been attained.
THE ARGONNE LABORATORY
8.27. The Argonne Laboratory was constructed early in 1943
outside Chicago. The site, originally intended for a pilot plant,
was later considered to be too near the city and was used for
reconstructing the so-called West Stands pile which was orig-
inally built on the University of Chicago grounds and which was
certainly innocuous. Under the direction of E. Fermi and his
colleagues, H. L. Anderson, W. H. Zinn, G. Weil, and others,
this pile has served as a prototype unit for studies of thermal
stability, controls, instruments, and shielding, and as a neutron
source for materials testing and neutron-physics studies. Further-
more, it has proved valuable as a training school for plant oper-
ators. More recently a heavy- water pile (see below) has been
constructed there.
8.28. The first Argonne pile, a graphite-uranium pile, need
not be described in detail. The materials and lattice structure
are nearly identical to those which were used for the original
140 Plutonium Problem to June 1945
West Stands pile. The pile is a cube; it is surrounded by a shield
and has controls and safety devices somewhat similar to those
used later at Clinton. It has no cooling system and is normally
run at a power level of only a few kilowatts. It has occasionally
been run at high-power levels for very brief periods. Considering
that it is merely a reconstruction of the first chain-reacting unit
ever built, it is amazing that it has continued in operation for
more than two years without developing any major troubles.
8.29. One of the most valuable uses of the Argonne pile has
been the measurement of neutron-absorption cross sections of a
great variety of elements which might be used in piles as struc-
tural members, etc., or which might be present in pile materials
as impurities. These measurements are made by observing the
change in the controls necessary to make kff equal to 1 .00 when
a known amount of the substance under study is inserted at a
definite position in the pile. The results obtained were usually
expressed in terms of "danger coefficients."
8.30. An opening at the top of the pile lets out a very uniform
beam of thermal neutrons that can be used for exponential-pile
experiments, for direct measurements of absorption cross sections,
for Wilson cloud chamber studies, etc.
8.31. An interesting phenomenon occurring at the top of the
pile is the production of a beam or flow of "cold" neutrons. If a
sufficient amount of graphite is interposed between the upper
surface of the pile and an observation point a few yards above,
the neutron energy distribution is found to correspond to a
temperature much lower than that of the graphite. This is pre-
sumed to be the result of a preferential transmission by the
crystalline graphite of the slowest ("coldest") neutrons, whose
quantum-mechanical wave-length is great compared to the
distance between successive planes in the graphite crystals.
8.32. More recently a pile using heavy water as moderator
was constructed in the Argonne Laboratory. The very high
intensity beam of neutrons produced by this pile has been found
well-suited to the study of "neutron optics," e.g., reflection and
refraction of neutron beams as by graphite.
Plutonium Problem to June 1945 141
8.33. A constant objective of the Argonne Laboratory has been
a better understanding of nuclear processes in uranium, neptun-
ium, and plutonium. Repeated experiments have been made to
improve the accuracy of constants such as thermal-fission cross
sections of U-235, U-238, and Pu-239, probabilities of non-fission
neutron absorption by each of these nuclei, and number of neu-
trons emitted per fission.
THE CLINTON PLANT
8.34. In Chapter VI we mentioned plans for a "pilot" plant
for production of plutonium to be built at the Clinton site in
Tennessee. By January 1943, the plans for this project were well
along; construction was started soon afterward. M. D. Whitaker
was appointed director of the Clinton Laboratories. The pilot-
plant plans were made cooperatively by du Pont and the Metal-
lurgical Laboratory; construction was carried out by du Pont;
plant operation was maintained by the University of Chicago
as part of the Metallurgical Project.
8.35. The main purposes of the Clinton plant were to produce
some plutonium and to serve as a pilot plant for chemical separa-
tion. As regards research, the emphasis at Clinton was on chem-
istry and on the biological effects of radiations. A large laboratory
was provided for chemical analysis, for research on purification
methods, for fission-product studies, for development of inter-
mediate-scale extraction and decontamination processes, etc.
Later a "hot laboratory," i.e., a laboratory for remotely-con-
trolled work on highly radioactive material, was provided. There
is also an instrument shop and laboratory that has been used very
actively. There are facilities for both clinical and experimental
work of the health division, which has been very active. There is
a small physics laboratory in which some important work was
done using higher neutron intensities than were available at the
Argonne Laboratory. The principal installations constructed at
the Clinton Laboratory site were the pile and the separation
plant; these are briefly described below.
142 Plutonium Problem to June 1945
THE CLINTON PILE
8.36. In any steadily operating pile the effective multiplication
factor k must be kept at 1, whatever the power level. The best
k^ that had been observed in a uranium-graphite lattice could
not be achieved in a practical pile because of neutron leakage,
cooling system, cylindrical channels for the uranium, protective
coating on the uranium, and other minor factors. Granted air-
cooling and a maximum safe temperature for the surface of the
uranium, a size of pile had to be chosen that could produce
1 ,000 kw. The effective k would go down with rising temperature
but not sufficiently to be a determining factor. Though a sphere
was the ideal shape, practical considerations recommended a
rectangular block.
8.37. The Clinton pile consists of a cube of graphite containing
horizontal channels filled with uranium. The uranium is in the
form of metal cylinders protected by gas-tight casings of alumi-
num. The uranium cylinders or slugs may be slid into the chan-
nels in the graphite; space is left to permit cooling air to flow past,
and to permit pushing the slugs out at the back of the pile when
they are ready for processing. Besides the channels for slugs there
are various other holes through the pile for control rods, instru-
ments, etc.
8.38. The Clinton pile was considerably larger than the first
pile at Chicago (see Chapter VI). More important than the
increased size of the Clinton pile were its cooling system, heavier
shields, and means for changing the slugs. The production goal
of the Clinton plant was set at a figure which meant that the pile
should operate at a power level of 1,000 kw.
8.39. The instrumentation and controls are identical in princi-
ple to those of the first pile. Neutron intensity in the pile is
measured by a BFs ionization chamber and is controlled by boron
steel rods that can be moved in and out of the pile, thereby vary-
ing the fraction of neutrons available to produce fission.
8.40. In spite of an impressive array of instruments and safety
devices, the most striking feature of the pile is the simplicity of
Plutonium Problem to June 1945 143
operation. Most of the time the operators have nothing to do
except record the readings of various instruments.
THE SEPARATION PLANT
8.41. Here, as at Hanford, the plutonium processes have to
be carried out by remote control and behind thick shields. The
separation equipment is housed in a series of adjacent cells having
heavy concrete walls. These cells form a continuous structure
(canyon) which is about 100 feet long and is two- thirds buried in
the ground. Adjacent to this canyon are the control rooms,
analytical laboratories, and a laboratory for further purification
of the plutonium after it has been decontaminated to the point
of comparative safety.
8.42. Uranium slugs that have been exposed in the pile are
transferred under water to the first of these cells and are then
dissolved. Subsequent operations are performed by pumping
solutions or slurries from one tank or centrifuge to another.
PERFORMANCE OF CLINTON PILE
8.43. The Clinton pile started operating on November 4, 1943,
and within a few days was brought up to a power level of 500 kw
at a maximum slug surface temperature of 110 C. Improvements
in the air circulation and an elevation of the maximum uranium
surface temperature to 150 C. brought the power level up to
about 800 kw, where it was maintained until the spring of 1944.
Starting at that time, a change was made in the distribution of
uranium, the change being designed to level out the power
distribution in the pile by reducing the amount of metal near the
center relative to that further out and thereby to increase the
average power level without anywhere attaining too high a tem-
perature. At the same time improvements were realized in the
sealing of the slug jackets, making it possible to operate the pile
at higher temperature. As a result, a power level of 1,800 kw was
attained in May 1944; this was further increased after the
installation of better fans in June 1944.
144 Plutonium Problem to June 1945
8.44. Thus the pile performance of June 1944 considerably
exceeded expectations. In ease of control, steadiness of operation,
and absence of dangerous radiation, the pile has been most
satisfactory. There have been very few failures attributable to
mistakes in design or construction.
8.45. The pile itself was simple both in principle and in
practice. Not so the plutonium-separation plant. The step from
the first chain-reacting pile to the Clinton pile was reasonably
predictable; but a much greater and more uncertain step was
required in the case of the separation process, for the Clinton
separation plant was designed on the basis of experiments using
only microgram amounts of plutonium.
8.46. Nevertheless, the separation process worked! The first
batch of slugs from the pile entered the separation plant on
December 20, 1943. By the end of January 1944, metal from the
pile was going to the separation plant at the rate of % ton per
day. By February 1 , 1 944, 1 90 mg of plutonium had been delivered
and by March 1, 1944, several grams had been delivered. Further-
more, the efficiency of recovery at the very start was about 50 per
cent, and by June 1 944 it was between 80 and 90 per cent.
8.47. During this whole period there was a large group of
chemists at Clinton working on improving the process and
developing it for Hanford. The Hanford problem differed from
that at Clinton in that much higher concentrations of plutonium
were expected. Furthermore, though the chemists were to be
congratulated on the success of the Clinton plant, the process
was complicated and expensive. Any improvements in yield or
decontamination or in general simplification were very much to
be sought.
8.48. Besides the proving of the pile and the separation plant
and the production of several grams of plutonium for experimental
use at Chicago, Clinton, and elsewhere, the Clinton Laboratories
have been invaluable as a training and testing center for Hanford,
for medical experiments, pile studies, purification studies, and
physical and chemical studies of plutonium and fission products.
8.49. As typical of the kind of problems tackled there and at
Plutonium Problem to June 1945 145
Chicago, the following problems listed in a single routine report
for May 1944 are pertinent:
Problems Closed Out during May 1944: Search for New Oxidizing
Agent, Effect of Radiation on Water and Aqueous Solutions,
Solubility of Plutonium Peroxide, Plutonium Compounds Suit-
able for Shipment, Fission Product Distribution in Plant Process
Solutions, Preliminary Process Design for Adsorption Extraction,
Adsorption Semi- Works Assistance, Completion of Adsorption
Process Design.
New Problems Assigned during May 1944: New Product Analysis
Method, Effect of Radiation on Graphite, Improvement in
Yield, New Pile Explorations, Waste Uranium Recovery,
Monitoring Stack Gases, Disposal of Active Waste Solutions,
Spray Cooling of X Pile, Assay Training Program, Standardiza-
tion of Assay Methods, Development of Assay Methods, Shielded
Apparatus for Process Control Assays, Cloud Chamber Experi-
ment, Alpha Particles from U-235, Radial Product Distribution,
Diffraction of Neutrons.
THE HANFORD PLANT
8.50. It is beyond the scope of this report to give any account
of the construction of the Hanford Engineer Works, but it is to
be hoped that the full story of this extraordinary enterprise and
the companion one, the Clinton Engineer Works, will be pub-
lished at some time in the future. The Hanford site was examined
by representatives of General Groves and of du Pont at the end
of 1 942, and use of the site was approved by General Groves after
he had inspected it personally. It was on the west side of the
Columbia River in central Washington north of Pasco. In the
early months of 1943 a 200-square-mile tract in this region was
acquired by the government (by lease or purchase) through the
Real Estate Division of the Office of the Chief of Engineers.
Eventually an area of nearly a thousand square miles was brought
under government control. At the time of acquisition of the land
there were a few farms and two small villages, Hanford and
Richland, on the site, which was otherwise sage-brush plains
146 Plutonium Problem to June 1945
and barren hills. On the 6th of April, 1943, ground was broken
for the Hanford construction camp. At the peak of activity in
1944, this camp was a city of 60,000 inhabitants, the fourth largest
city in the state. Now, however, the camp is practically deserted
as the operating crew is housed at Richland.
8.51. Work was begun on the first of the Hanford production
piles on June 7, 1943, and operation of the first pile began in
September 1944. The site was originally laid out for five piles,
but the construction of only three has been undertaken. Besides
the piles, there are, of course, plutonium separation plants,
pumping stations and water-treatment plants. There is also a
low-power chain-reacting pile for material testing. Not only are
the piles themselves widely spaced for safety several miles apart
but the separation plants are well away from the piles and
from each other. All three piles were in operation by the summer
of 1945.
CANNING AND CORROSION
8.52. No one who lived through the period of design and con-
struction of the Hanford plant is likely to forget the "canning"
problem, i.e., the problem of sealing the uranium slugs in pro-
tective metal jackets. On periodic visits to Chicago the writer
could roughly estimate the state of the canning problem by the
atmosphere of gloom or joy to be found around the laboratory.
It was definitely not a simple matter to find a sheath that would
protect uranium from water corrosion, would keep fission products
out of the water, would transmit heat from the uranium to the
water, and would not absorb too many neutrons. Yet the failure
of a single can might conceivably require shut-down of an entire
operating pile.
8.53. Attempts to meet the stringent requirements involved
experimental work on electroplating processes, hot-dipping
processes, cementation-coating processes, corrosion-resistant alloys
of uranium, and mechanical jacketing or canning processes.
Mechanical jackets or cans of thin aluminum were feasible from
the nuclear-physics point of view and were chosen early as the
Plutonium Problem to June 1945 147
most likely solution of the problem. But the problem of getting
a uniform, heat-conducting bond between the uranium and the
surrounding aluminum, and the problem of effecting a gas-tight
closure for the can both proved very troublesome. Development
of alternative methods had to be carried along up to the last
minute, and even up to a few weeks before it was time to load
the uranium slugs into the pile there was no certainty that any
of the processes under development would be satisfactory. A
final minor but apparently important modification in the pre-
ferred canning process was adopted in October 1944, after the
first pile had begun experimental operation. By the summer of
1945, there had been no can failure reported.
PRESENT STATUS OF THE HANFORD PLANTS
8.54. During the fall of 1944 and the early months of 1945
the second and third Hanford piles were finished and put into
operation, as were the additional chemical separation plants.
There were, of course, some difficulties; however, none of the
fears expressed as to canning failure, film formation in the water
tubes, or radiation effects in the chemical processes, have turned
out to be justified. As of early summer 1945 the piles are operat-
ing at designed power, producing plutonium, and heating the
Columbia River.* The chemical plants are separating the plu-
tonium from the uranium and from the fission products with
better efficiency than had been anticipated. The finished product
is being delivered. How it can be used is the subject of Chapter
XII.
THE WORK ON HEAVY WATER
8.55. In previous chapters there have been references to the
advantages of heavy water as a moderator. It is more effective
than graphite in slowing down neutrons and it has a smaller
neutron absorption than graphite. It is therefore possible to build
* The actual rise in temperature is so tiny that no effect on fish life could
be expected. To make doubly sure, this expectation was confirmed by an
elaborate series of experiments.
148 Plutonium Problem to June 1945
a chain-reacting unit with uranium and heavy water and thereby
to attain a considerably higher multiplication factor, k, and a
smaller size than is possible with graphite. But one must have
the heavy water.
8.56. In the spring of 1943 the Metallurgical Laboratory
decided to increase the emphasis on experiments and calculations
aimed at a heavy- water pile. To this end a committee was set up
under E. Wigner, a group under H. C. Vernon was transferred
from Columbia to Chicago, and H. D. Smyth, who had just
become associate director of the Laboratory, was asked to take
general charge.
8.57. The first function of this group was to consider in what
way heavy water could best be used to insure the overall success
of the Metallurgical Project, taking account of the limited produc-
tion schedule for heavy water that had been already authorized.
8.58. It became apparent that the production schedule was
so low that it would take two years to produce enough heavy
water to "moderate" a fair-sized pile for plutonium production.
On the other hand, there might be enough heavy water to moder-
ate a small "laboratory" pile, which could furnish information
that might be valuable. In any event, during the summer of 1943
so great were the uncertainties as to the length of the war and
as to the success of the other parts of the DSM project that a
complete study of the possibilities of heavy-water piles seemed
desirable. Either the heavy-water production schedule might be
stepped up or the smaller, experimental pile might be built. An
intensive study of the matter was made during the summer of
1943 but in November it was decided to curtail the program
and construction was limited to a 250-kw pile located at the
Argonne site.
THE ARGONNE HEAVY- WATER PILE
8.59. Perhaps the most striking aspect of the uranium and
heavy-water pile at the Argonne is its small size. Even with its
surrounding shield of concrete it is relatively small compared
to the uranium-graphite piles.
Plutonium Problem to June 1945 149
8.60. By May 15, 1944, the Argonne uranium and heavy-water
pile was ready for test. With the uranium slugs in place, it was
found that the chain reaction in the pile became self sustaining
when only three fifths of the heavy water had been added. The
reactivity of the pile was so far above expectations that it would
have been beyond the capacity of the control rods to handle if
the remainder of the heavy water had been added. To meet this
unusual and pleasant situation some of the uranium was removed
and extra control rods were added.
8.61. With these modifications it was possible to fill the tank
to the level planned. By July 4, 1944, W. H. Zinn reported that
the pile was running satisfactorily at 190 kw, and by August 8,
1 944, he reported that it was operating at 300 kw.
8.62. In general the characteristics of this pile differed slightly
from those of comparable graphite piles. This pile takes several
hours to reach equilibrium. It shows small (less than 1 per cent)
but sudden fluctuations in power level, probably caused by
bubbles in the water. It cannot be shut down as completely or
as rapidly as the graphite pile because of the tendency of delayed
gamma rays to produce (from the heavy water) additional
neutrons. As anticipated, the neutron density at the center is
high. The shields, controls, heat exchanger, etc., have operated
satisfactorily.
THE HEALTH DIVISION
8.63. The major objective of the health group was in a sense a
negative one, to insure that no one concerned suffered serious
injury from the peculiar hazards of the enterprise. Medical case
histories of persons suffering serious injury or death resulting
from radiation were emphatically not wanted. The success of the
health division in meeting these problems was remarkable. Even
in the research group where control is most difficult, cases show-
ing even temporary bad effects were extremely rare. Factors of
safety used in plant design and operation are so great that the
hazards of the home and the family car are far greater for the
personnel than any arising from the plants.
150 Plutonium Problem to June 1945
8.64. To achieve its objective the health group worked along
three major lines:
(1) Adoption of pre-employment physical examinations and
frequent re-examinations, particularly of those exposed to
radiation.
(2) Setting of tolerance standards for radiation doses and
development of instruments measuring exposure of personnel;
giving advice on shielding, etc. ; continually measuring radiation
intensities at various locations in the plants; measuring con-
tamination of clothes, laboratory desks, waste water, the atmos-
phere, etc.
(3) Carrying out research on the effects of direct exposure of
persons and animals to various types of radiation, and on the
effects of ingestion and inhalation of the various radioactive or
toxic materials such as fission products, plutonium and uranium.
ROUTINE EXAMINATIONS
8.65. The white blood-corpuscle count was used as the princi-
pal criterion as to whether a person suffered from overexposure
to radiation. A number of cases of abnormally low counts were
observed and correlated with the degree of overexposure. Indi-
viduals appreciably affected were shifted to other jobs or given
brief vacations; none has shown permanent ill effects.
8.66. At the same time it was recognized that the white blood-
corpuscle count is not an entirely reliable criterion. Some work
on animals indicated that serious damage might occur before
the blood count gave any indication of danger. Accordingly,
more elaborate blood tests were made on selected individuals
and on experimental animals in the hope of finding a test that
would give an earlier warning of impending injury.
INSTRUMENTS FOR RADIATION MEASUREMENTS
8.67. The Health Division had principal responsibility for the
development of pocket meters for indicating the extent of ex-
posure of persons. The first of these instruments was a simple
Plutonium Problem to June 1945 151
electroscope about the size and shape of a fountain pen. Such
instruments were electrostatically charged at the start of each
day and were read at the end of the day. The degree to which
they became discharged indicated the total amount of ionizing
radiation to which they had been exposed. Unfortunately they
were none too rugged and reliable, but the error of reading was
nearly always in the right direction i.e., in the direction of over-
stating the exposure. At an early date the practice was estab-
lished of issuing two of these pocket meters to everyone entering
a dangerous area. A record was kept of the readings at the time
of issuance and also when the meters were turned in. The meters
themselves were continually although gradually improved. The
Health Division later introduced "film badges," small pieces of
film worn in the identification badge, the films being periodically
developed and examined for radiation blackening. These instru-
ments for individuals such as the pocket meter and film badge
were extra and probably unnecessary precautions. In permanent
installations the shielding alone normally affords complete safety.
Its effect is under frequent survey by either permanently installed
or portable instruments.
8.68. The Health Division cooperated with the Physics Division
in the development and use of various other instruments. There
was "Sneezy" for measuring the concentration of radioactive
dust in the air and "Pluto" for measuring a-emitting contamina-
tion (usually plutonium) of laboratory desks and equipment.
Counters were used to check the contamination of laboratory
coats before and after the coats were laundered. At the exit gates
of certain laboratories concealed counters sounded an alarm
when someone passed whose clothing, skin or hair was con-
taminated. In addition, routine inspections of laboratory areas
were made.
8.69. One of the studies made involved meteorology. It be-
came essential to know whether the stack gases (at Clinton and
at Hanford) would be likely to spread radioactive fission products
in dangerous concentrations. Since the behavior of these gases
is very dependent on the weather, studies were made at both
152 Plutonium Problem to June 1945
sites over a period of many months, and satisfactory stack opera-
tion was specified.
RESEARCH
8.70. Since both the scale and the variety of the radiation
hazards in this enterprise were unprecedented, all reasonable
precautions were taken; but no sure means were at hand for
determining the adequacy of the precautions. It was essential to
supplement previous knowledge as completely as possible. For
this purpose, an extensive program of animal experimentation
was carried out along three main lines: (1) exposure to neutron,
alpha, beta and gamma radiation; (2) ingestion of uranium,
plutonium and fission products; (3) inhalation of uranium,
plutonium and fission products. Under the general direction of
Dr. Stone these experiments were carried out at Chicago, Clinton
and the University of California principally by Dr. Cole and Dr.
Hamilton. Extensive and valuable results were obtained.
SUMMARY
8.71. Both space and security restrictions prevent a detailed
report on the work of the laboratories and plants concerned with
plutonium production.
8.72. Two types of neutron absorption are fundamental to the
operation of the plant: one, neutron absorption in U-235 result-
ing in fission, maintains the chain reaction as a source of neutrons;
the other, neutron absorption in U-238 leads to the formation of
plutonium, the desired product.
8.73. The course of a nuclear chain reaction in a graphite-
moderated heterogeneous pile can be described by following a
single generation of neutrons. The original fast neutrons are
slightly increased in number by fast fission, reduced by resonance
absorption in U-238 and further reduced by absorption at thermal
energies in graphite and other materials and by escape; the
remaining neutrons, which have been slowed in the graphite,
cause fission in U-235, producing a new generation of fast
neutrons similar to the previous generation.
Plutonium Problem to June 7945 153
8.74. The product, plutonium, must be separated by chemical
processes from a comparable quantity of fission products and a
much larger quantity of uranium. Of several possible separation
processes the one chosen consists of a series of reactions including
precipitating with carriers, dissolving, oxidizing and reducing.
8.75. The chain reaction was studied at low power at the
Argonne Laboratory beginning early in 1943. Both chain reaction
and chemical separation processes were investigated at the
Clinton Laboratories beginning in November 1943, and an
appreciable amount of plutonium was produced there.
8.76. Construction of the main production plant at Hanford,
Washington, was begun in 1943 and the first large pile went into
operation in September 1 944. The entire plant was in operation
by the summer of 1945 with all chain-reacting piles and chemical-
separation plants performing better than had been anticipated.
8.77. Extensive studies were made on the use of heavy water
as a moderator and an experimental pile containing heavy water
was built at the Argonne Laboratory. Plans for a production
plant using heavy water were given up.
8.78. The Health Division was active along three main lines:
(1) medical examination of personnel; (2) advice on radiation
hazards and constant check on working conditions; (3) research
on the effects of radiation. The careful planning and exhaustive
research work of this division have resulted in an outstanding
health record at Hanford and elsewhere in the project.
CHAPTER IX. GENERAL DISCUSSION OF THE
SEPARATION OF ISOTOPES
INTRODUCTORY NOTE
9.1. The possibility of producing an atomic bomb of U-235
was recognized before plutonium was discovered. Because it was
appreciated at an early date that the separation of the uranium
isotopes would be a direct and major step toward making such a
bomb, methods of separating uranium isotopes have been under
scrutiny for at least six years. Nor was attention confined to
uranium since it was realized that the separation of deuterium
was also of great importance. In the present chapter the general
problems of isotope separation will be discussed; later chapters
will take up the specific application of various processes.
FACTORS AFFECTING THE SEPARATION
OF ISOTOPES
9.2. By definition, the isotopes of an element differ in mass but
not in chemical properties. More precisely, although the nuclear
masses and structures differ, the nuclear charges are identical
and therefore the external electronic structures are practically
identical. For most practical purposes, therefore, the isotopes of
an element are separable only by processes depending on the
nuclear mass.
9.3. It is well known that the molecules of a gas or liquid are
in continual motion and that their average kinetic energy depends
only on the temperature, not on the chemical properties of the
molecules. Thus in a gas made up of a mixture of two isotopes
the average kinetic energy of the light molecules and of the heavy
ones is the same. Since the kinetic energy of a molecule is J^ mv 2 ,
where m is the mass and v the speed of the molecule, it is appar-
ent that on the average the speed of a lighter molecule must be
154
The Separation of Isotopes 155
greater than that of a heavier molecule. Therefore, at least in
principle, any process depending on the average speed of mole-
cules can be used to separate isotopes. Unfortunately, the average
speed is inversely proportional to the square root of the mass so
that the difference is very small for the gaseous compounds of
the uranium isotopes. Also, although the average speeds differ, the
ranges of speed show considerable overlap. In the case of the gas
uranium hexafluoride, for example, over 49 per cent of the light
molecules have speeds as low as those of 50 per cent of the heavy
molecules.
9.4. Obviously there is no feasible way of applying mechanical
forces directly to molecules individually; they cannot be poked
with a stick or pulled with a string. But they are subject to gravi-
tational fields and, if ionized, may be affected by electric and
magnetic fields. Gravitational forces are, of course, proportional
to the mass. In a very high vacuum U-235 atoms and U-238 atoms
would fall with the same acceleration, but just as a feather and
a stone fall at very different rates in air where there are frictional
forces resisting motion, there may be conditions under which a
combination of gravitational and opposing intermolecular forces
will tend to move heavy atoms differently from light ones.
Electric and magnetic fields are more easily controlled than
gravitational fields or "pseudogravitational" fields (i.e., centrif-
ugal-force fields) and are very effective in separating ions of
differing masses.
9.5. Besides gravitational or electromagnetic forces, there are,
of course, interatomic and intermolecular forces. These forces
govern the interaction of molecules and thus affect the rates of
chemical reactions, evaporation processes, etc. In general, such
forces will depend on the outer electrons of the molecules and not
on the nuclear masses. However, whenever the forces between
separated atoms or molecules lead to the formation of new
molecules, a mass effect (usually very small) does appear. In
accordance with quantum-mechanical laws, the energy levels
of the molecules are slightly altered, and differently for each
isotope. Such effects do slightly alter the behavior of two isotopes
156 The Separation of Isotopes
in certain chemical reactions, as we shall see, although the dif-
ference in behavior is far smaller than the familiar differences of
chemical behavior between one element and another.
9.6. These, then, are the principal factors that may have to be
considered in devising a separation process: equality of average
thermal kinetic energy of molecules at a given temperature,
gravitational or centrifugal effects proportional to the molecular
masses, electric or magnetic forces affecting ionized molecules,
and interatomic or intermolecular forces. In some isotope-
separation processes only one of these effects is involved and the
overall rate of separation can be predicted. In other isotope-
separation processes a number of these effects occur simultane-
ously so that prediction becomes difficult.
CRITERIA FOR APPRAISING A SEPARATION PROCESS
9.7. Before discussing particular processes suitable for isotope
separation, we should know what is wanted. The major criteria
to be used in judging an isotope-separation process are as follows.
SEPARATION FACTOR
9.8. The separation factor, sometimes known as the enrich-
ment or fractionating factor of a process, is the ratio of the relative
concentration of the desired isotope after processing to its relative
concentration before processing. Defined more precisely: if, before
the processing, the numbers of atoms of the isotopes of mass
number mi and m 2 are ni and n 2 respectively (per gram of the
isotope mixture) and if, after the processing, the corresponding
numbers are n^ and n' 2 , then the separation factor is:
ni/n a
This definition may be applied to one stage of a separation plant
or to an entire plant consisting of many stages. We are usually
interested either in the "single stage" separation factor or in the
"overall" separation factor of the whole process. If r is only
slightly greater than unity, as is often the case for a single stage,
The Separation of Isotopes 1 57
the number r 1 is sometimes more useful than r. The quantity
r 1 is called the enrichment factor. In natural uranium
mi = 235, m z = 238, and ni/n 2 = K40 approximately, but
in 90 per cent U-235, n\/n' 2 %. Consequently in a process
producing 90 per cent U-235 from natural uranium the overall
value of r must be about 1,260.
YIELD
9.9. In nearly every process a high separation factor means a
low yield, a fact that calls for continual compromise. Unless
indication is given to the contrary, we shall state yields in terms
of U-235. Thus a separation device with a separation factor of
2 that is, nj/n'a = Mo and a yield of one gram a day is one that,
starting from natural uranium, produces, in one day, material
consisting of 1 gram of U-235 mixed with 70 grams of U-238.
HOLD-UP
9.10. The total amount of material tied up in a separation
plant is called the "hold-up." The hold-up may be very large
in a plant consisting of many stages.
START-UP TIME
9.11. In a separation plant having large hold-up, a long time
perhaps weeks or months is needed for steady operating con-
ditions to be attained. In estimating time schedules this "start-
up" or "equilibrium" time must be added to the time of
construction of the plant.
EFFICIENCY
9.12. If a certain quantity of raw material is fed into a separa-
tion plant, some of the material will be enriched, some impover-
ished, some unchanged. Parts of each of these three fractions will
be lost and parts recovered. The importance of highly efficient
recovery of the enriched material is obvious. In certain processes
the amount of unchanged material is negligible, but in others,
notably in the electromagnetic method to be described below, it
158 The Separation of Isotopes
is the largest fraction and consequently the efficiency with which
it can be recovered for recycling is very important. The impor-
tance of recovery of impoverished material varies widely, depend-
ing very much on the degree of impoverishment. Thus in general
there are many different efficiencies to be considered.
COST
9.13. As in all parts of the uranium project, cost in time was
more important than cost in money. Consequently a number of
large-scale separation plants for U-235 and deuterium were built
at costs greater than would have been required if construction
could have been delayed for several months or years until more
ideal processes were worked out.
SOME SEPARATION PROCESSES
GASEOUS DIFFUSION
9.14. As long ago as 1896 Lord Rayleigh showed that a mixture
of two gases of different atomic weight could be partly separated
by allowing some of it to diffuse through a porous barrier into an
evacuated space. Because of their higher average speed the
molecules of the light gas diffuse through the barrier faster so
that the gas which has passed through the barrier (i.e., the
"diffusate") is enriched in the lighter constituent and the residual
gas which has not passed through the barrier is impoverished
in the lighter constituent. The gas most highly enriched in the
lighter constituent is the so-called "instantaneous diffusate"; it
is the part that diffuses before the impoverishment of the residue
has become appreciable. If the diffusion process is continued
until nearly all the gas has passed through the barrier, the average
enrichment of the diffusate naturally diminishes. In the next
chapter we shall consider these phenomena more fully.
Here we shall merely point out that, on the assumption that the
diffusion rates are inversely proportional to the square roots of
the molecular weights the separation factor for the instantaneous
diffusate, called the "ideal separation factor" a, is given by
The Separation of Isotopes 159
M,
where MI is the molecular weight of the lighter gas and M 2 that
of the heavier. Applying this formula to the case of uranium will
illustrate the magnitude of the separation problem. Since uranium
itself is not a gas, some gaseous compound of uranium must be
used. The only one obviously suitable is uranium hexafluoride,
UFe, which has a vapor pressure of one atmosphere at a tem-
perature of 56 C. Since fluorine has only one isotope, the two
important uranium hexafluorides are U 236 F 6 and U 238 F 6 ; their
molecular weights are 349 and 352. Thus, if a small fraction of a
quantity of uranium hexafluoride is allowed to diffuse through a
porous barrier, the diffusate will be enriched in U 235 Fe by a factor
= 1.0043
which is a long way from the 1,260 required (see paragraph 9.8.).
9.15. Such calculations might make it seem hopeless to sepa-
rate isotopes (except, perhaps, the isotopes of hydrogen) by
diffusion processes. Actually, however, such methods may be
used successfully even for uranium. It was the gaseous diffusion
method that F. W. Aston used in the first partial separation of
isotopes (actually the isotopes of neon). Later G. Hertz and
others, by operating multi-stage recycling diffusion units, were
able to get practically complete separation of the neon isotopes.
Since the multiple-stage recycling system is necessary for nearly
all separation methods, it will be described in some detail immedi-
ately following introductory remarks on the various methods to
which it is pertinent.
FRACTIONAL DISTILLATION
9.16. The separation of compounds of different boiling points,
i.e., different vapor pressures, by distillation is a familiar indus-
trial process. The separation of alcohol and water (between
which the difference in boiling point is in the neighborhood of
160 The Separation of Isotopes
20 C.) is commonly carried out in a simple still using but a
single evaporator and condenser. The condensed material (con-
densate) may be collected and redistilled a number of times if
necessary. For the separation of compounds of very nearly the
same boiling point it would be too laborious to carry out the
necessary number of successive evaporations and condensations
as separate operations. Instead, a continuous separation is
carried out in a fractionating tower. Essentially the purpose of
a fractionating tower is to produce an upward-directed stream
of vapor and a downward-directed stream of liquid, the two
streams being in intimate contact and constantly exchanging
molecules. The molecules of the fraction having the lower boiling
point have a relatively greater tendency to get into the vapor
stream and vice versa. Such counter-current distillation methods
can be applied to the separation of light and heavy water, which
differ in boiling point by 1 .4 C.
GENERAL APPLICATION OF COUNTERCURRENT FLOW
9.17. The method of counter current flow is useful not only in
two-phase (liquid-gas) distillation processes, but also in other
separation processes such as those involving diffusion resulting
from temperature variations (gradients) within one-phase systems
or from centrifugal forces. The countercurrents may consist of
two gases, two liquids, or one gas and one liquid.
THE CENTRIFUGE
9.18. We have pointed out that gravitational separation of
two isotopes might occur since the gravitational forces tending
to move the molecules downward are proportional to the mole-
cular weights, and the intermolecular forces tending to resist
the downward motion depend on the electronic configuration,
not on the molecular weights. Since the centrifuge is essentially
a method of applying pseudogravitational forces of large magni-
tude, it was early considered as a method for separating isotopes.
However, the first experiments with centrifuges failed. Later
The Separation of Isotopes 161
development of the high speed centrifuge by J. W. Beams and
others led to success. H. C. Urey suggested the use of tall cylin-
drical centrifuges with countercurrent flow; such centrifuges have
been developed successfully.
9.19. In such a countercurrent centrifuge there is a downward
flow of vapor in the outer part of the rotating cylinder and an
upward flow of vapor in the central or axial region. Across the
interface region between the two currents there is a constant
diffusion of both types of molecules from one current to the other,
but the radial force field of the centrifuge acts more strongly on
the heavy molecules than on the light ones so that the concentra-
tion of heavy ones increases in the peripheral region and decreases
in the axial region, and vice versa for the lighter molecules.
9.20. The great appeal of the centrifuge in the separation of
heavy isotopes like uranium is that the separation factor depends
on the difference between the masses of the two isotopes, not on
the square root of the ratio of the masses as in diffusion methods.
THERMAL DIFFUSION METHOD
9.21. The kinetic theory of gases predicts the extent of the
differences in the rates of diffusion of gases of different molecular
weights. The possibility of accomplishing practical separation of
isotopes by thermal diffusion was first suggested by theoretical
studies of the details of molecular collisions and of the forces
between molecules. Such studies made by Enskog and by Chap-
man before 1920 suggested that if there were a temperature
gradient in a mixed gas there would be a tendency for one type
of molecule to concentrate in the cold region and the other in
the hot region. This tendency depends not only on the molecular
weights but also on the forces between the molecules. If the gas
is a mixture of two isotopes, the heavier isotope may accumulate
at the hot region or the cold region or not at all, depending on
the nature of the intermolecular forces. In fact, the direction of
separation may reverse as the temperature or relative concen-
tration is changed.
1 62 The Separation of Isotopes
9.22. Such thermal diffusion effects were first used to separate
isotopes by H. Clusius and G. Dickel in Germany in 1938. They
built a vertical tube containing a heated wire stretched along
the axis of the tube and producing a temperature difference of
about 600 C. between the axis and the periphery. The effect was
twofold. In the first place, the heavy isotopes (in the substances
they studied) became concentrated near the cool outer wall, and
in the second place, the cool gas on the outside tended to sink
while the hot gas at the axis tended to rise. Thus thermal con-
vection set up a countercurrent flow, and thermal diffusion
caused the preferential flow of the heavy molecules outward
across the interface between the two currents.
9.23. The theory of thermal diffusion in gases is intricate
enough; that of thermal diffusion in liquids is practically impos-
sible. A separation effect does exist, however, and has been used
successfully to separate the light and heavy uranium hexafluorides.
CHEMICAL EXCHANGE METHOD
9.24. In the introduction to this chapter we pointed out that
there was some reason to hope that isotope separation might be
accomplished by ordinary chemical reactions. It has in fact been
found that in simple exchange reactions between compounds of
two different isotopes the so-called equilibrium constant is not
exactly one, and thus that in reactions of this type separation
can occur. For example, in the catalytic exchange of hydrogen
atoms between hydrogen gas and water, the water contains
between three and four times as great a concentration of deu-
terium as the hydrogen gas in equilibrium with it. With hydrogen
and water vapor the effect is of the same general type but equilib-
rium is more rapidly established. It is possible to adapt this
method to a continuous countercurrent flow arrangement like
that used in distillation, and such arrangements are actually in
use for production of heavy water. The general method is well
understood, and the separation effects are known to decrease in
general with increasing molecular weight, so that there is but a
The Separation of Isotopes 163
small chance of applying this method successfully to heavy iso-
topes like uranium.
ELECTROLYSIS METHOD
9.25. The electrolysis method of separating isotopes resulted
from the discovery that the water contained in electrolytic cells
used in the regular commercial production of hydrogen and
oxygen has an increased concentration of heavy water molecules.
A full explanation of the effect has not yet been worked out.
Before the war practically the entire production of heavy hydro-
gen was by the electrolysis method. By far the greatest production
was in Norway, but enough for many experimental purposes had
been made in the United States.
STATISTICAL METHODS IN GENERAL
9.26. The six methods of isotope separation we have described
so far (diffusion, distillation, centrifugation, thermal diffusion,
exchange reactions, and electrolysis) have all been tried with some
degree of success on either uranium or hydrogen or both. Each
of these methods depends on small differences in the average
behavior of the molecules of different isotopes. Because an average
is by definition a statistical matter, all such methods depending
basically on average behavior are called statistical methods.
9.27. With respect to the criteria set up for judging separation
processes the six statistical methods are rather similar. In every
case the separation factor is small so that many successive stages
of separation are required. In most cases relatively large quantities
of material can be handled in plants of moderate size. The hold-
up and starting-time values vary considerably but are usually high.
The similarity of the six methods renders it inadvisable to make
final choice of method without first studying in detail the particu-
lar isotope, production rate, etc., wanted. Exchange reaction
and electrolysis methods are probably unsuitable in the case of
uranium, and no distillation scheme for uranium has survived.
All of the other three methods have been developed with varying
degrees of success for uranium, but are not used for hydrogen.
164
The Separation of Isotopes
THE ELECTROMAGNETIC METHOD
AND ITS LIMITATIONS
9.28. The existence of non-radioactive isotopes was first
demonstrated during the study of the behavior of ionized gas
molecules moving through electric and magnetic fields. It is just
ION SOURCE
SLIT S
SLIT
COLLECTOR
HEAVY
MAGNETIC FIELD PERPENDICULAR
TO PLANE OF DRAWING
such fields that form the basis of the so-called mass spectrographic
or electromagnetic method of separating isotopes. This method
is the best available for determining the relative abundance of
many types of isotope. The method is used constantly in checking
the results of the uranium isotope separation methods we have
already described. The reason the method is so valuable is that
it can readily effect almost complete separation of the isotopes
very rapidly and with small hold-up and short start-up time. If
The Separation of Isotopes 165
this is so, it may well be asked why any other method of separa-
tion is considered. The answer is that an ordinary mass spectro-
graph can handle only very minute quantities of material, usually
of the order of fractions of a microgram per hour.
9.29. To understand the reasons for this limitation in the yield,
we shall outline the principle of operation of a simple type of
mass spectrograph first used by A. J. Dempster in 1918. Such an
instrument is illustrated schematically in the drawing on p. 164.
The gaseous compound to be separated is introduced in the
ion source, where some of its molecules are ionized in an electric
discharge. Some of these ions go through the slit SL Between
Si and 82 they are accelerated by an electric field which gives
them all practically the same kinetic energy, thousands of times
greater than their average thermal energy. Since they now all
have practically the same kinetic energy, the lighter ions must
have less momenta than the heavy ones. Entering the magnetic
field at the slit 82 all the ions will move perpendicular to the
magnetic field in semi-circular paths of radii proportional to
their momenta. Therefore the light ions will move in smaller
semicircles than the heavy, and with proper positioning of the
collector, only the light ions will be collected.
9.30. Postponing detailed discussion of such a separation
device, we may point out the principal considerations that limit
the amount of material that passes through it. They are three-
fold: First, it is difficult to produce large quantities of gaseous
ions. Second, a sharply limited ion beam is usually employed (as
in the case shown) so that only a fraction of the ions produced
are used. Third, too great densities of ions in a beam can cause
space-charge effects which interfere with the separating action.
Electromagnetic methods developed before 1941 had very high
separation factors but very low yields and efficiencies. These were
the reasons which before the summer of 1941 led the Uranium
Committee to exclude such methods for large-scale separation
of U-235. (See Paragraph 4:31.) Since that time it has been shown
that the limitations are not insuperable. In fact, the first appreci-
able-size samples of pure U-235 were produced by an electro-
magnetic separator, as will be described in a later chapter.
1 66 The Separation of Isotopes
OTHER ISOTOPE-SEPARATION METHODS
9.31. In addition to the isotope-separation methods described
above, several other methods have been tried. These include the
ionic mobility method, which, as the name implies, depends on
the following fact: In an electrolytic solution two ions which are
chemically identical but of different mass progress through the
solution at different rates under the action of an electric field.
However, the difference of mobility will be small and easily
obscured by disturbing effects. A. K. Brewer of the Bureau of
Standards reported that he was able to separate the isotopes of
potassium by this method. Brewer also obtained some interesting
results with an evaporation method. Two novel electromagnetic
methods, the isotron and the ionic centrifuge, are described in
Chapter XI. The isotron produced a number of fair-size samples
of partly separated uranium. The ionic centrifuge also produced
some uranium samples showing separation, but its action was
erratic.
CASCADES AND COMBINED PROCESSES
9.32. In all the statistical methods of separating isotopes many
successive stages of separation are necessary to get material that
is 90 per cent or more U-235 or deuterium. Such a series of
successive separating stages is called a cascade if the flow is
continuous from one stage to the next. (A fractionating tower of
separate plates such as has been described is an example of a
simple cascade of separating units.) A complete analysis of the
problems of a cascade might be presented in general terms.
Actually it has been worked out by R. P. Feynman of Princeton
and others for a certain type of electromagnetic separator and by
K. Cohen and I. Kaplan of Columbia, by M. Benedict and A. M.
Squires of the Kellex Corporation and others for diffusion proc-
esses. At present we shall make only two points about multiple-
stage or "cascade" plants.
9.33. The first point is that there must be recycling. Con-
sidering a U-235 separation plant, the material fed into any stage
The Separation of Isotopes 167
above the first has already been enriched in U-235. Part of this
feed material may be further enriched in passing through the
stage under consideration. The remainder will typically become
impoverished but not so much impoverished as to be valueless.
It must be returned to an earlier stage and recycled. Even the
impoverished material from the first (least enriched) stage may be
worth recycling; some of the U-235 it still contains may be re-
covered (stripped).
9.34. The second point is that the recycling problem changes
greatly at the higher (more enriched) stages. Assuming steady
stage operation, we see that the net flow of uranium through the
first stage must be at least 140 times as great as through the last
stage. The net flow in any given stage is proportional to the rela-
tive concentration of U-238 and thus decreases with the number of
stages passed. Since any given sample of material is recycled many
times, the amount of material processed in any stage is far greater
than the net flow through that stage but is proportional to it.
9.35. We mention these points to illustrate a phase of the
separation problem that is not always obvious, namely, that the
separation process which is best for an early stage of separation
is not necessarily best for a later stage. Factors such as those we
have mentioned differ not only from stage to stage but from
process to process. For example, recycling is far simpler in a
diffusion plant than in an electromagnetic plant. A plant com-
bining two or more processes may well be the best to accomplish
the overall separation required. In the lower (larger) stages the
size of the equipment and the power required for it may deter-
mine the choice of process. In the higher (smaller) stages these
factors are outweighed by convenience of operation and hold-up
time, which may point to a different process.
THE HEAVY WATER PLANTS; THE CENTRIFUGE
PILOT PLANT
9.36. The next two chapters are devoted to descriptions of the
three methods used for large-scale separation of the uranium
isotopes. These are the only isotope-separation plants that have
168 The Separation of Isotopes
turned out to be of major importance to the project up to the
present time. At an earlier stage it seemed likely that the centri-
fuge might be the best method for separating the uranium isotopes
and that heavy water would be needed as a moderator. We shall
describe briefly the centrifuge pilot plant and the heavy water
production plants.
THE HEAVY WATER PLANTS
9.37. Two methods were used for the concentration of deu-
terium. These were the fractional distillation of water and the
hydrogen-water exchange reaction method.
9.38. The first of these follows well established fractional
distillation methods except that very extensive distillation is
required because of the slight difference in boiling point of light
and heavy water. Also, because of this same small difference, the
amount of steam required is very large. The method is very
expensive because of these factors, but plants could be con-
structed with a minimum of development work. Plants were
started by du Pont in January 1943, and were put into operation
about January 1 944.
9.39. The second method for the preparation of heavy water
depends upon the catalytic exchange of deuterium between
hydrogen gas and water. When such an exchange is established
by catalysts, the concentration of the deuterium in the water is
greater than that in the gas by a factor of about three as we have
already seen.
9.40. In this process water is fed into a tower and flows counter-
currently to hydrogen and steam in an intricate manner. At the
bottom of the tower the water is converted to hydrogen gas and
oxygen gas in electrolytic cells and the hydrogen is fed back to
the bottom of the tower mixed with steam. This steam and
hydrogen mixture passes through beds of catalyst and bubbles
through the downflowing water. Essentially, part of the deu-
terium originally in the hydrogen concentrates in the steam and
then is transferred to the downflowing water. The actual plant
consists of a cascade of towers with the largest towers at the feed
The Separation of Isotopes 169
end and the smallest towers at the production end. Such a cascade
follows the same general principle as those discussed above in
connection with separation problems in general. This process
required the securing of very active catalysts for the exchange
reactions. The most effective catalyst of this type was discovered
by H. S. Taylor at Princeton University, while a second, less
active catalyst was discovered by A. von Grosse. In the develop-
ment of these catalysts R. H. Crist of Columbia University made
the necessary determinations of physical constants and H. R.
Arnold of du Pont did the development work on one of the
catalysts.
9.41. This process was economical in operation. The plant was
placed at the works of the Consolidated Mining & Smelting Co.,
at Trail, British Columbia, Canada, because of the necessity of
using electrolytic hydrogen. The construction of the plant was
under the direction of E. V. Murphree and F. T. Barr of the
Standard Oil Development Co.
THE CENTRIFUGE PILOT PLANT
9.42. For a long time in the early days of the project the gase-
ous diffusion method and the centrifuge method were considered
the two separation methods most likely to succeed with uranium.
Both were going to be difficult to realize on a large scale. After
the reorganization in December 1941 research and development
on the centrifuge method continued at the University of Virginia
and at the Standard Oil Development Company's laboratory at
Bayway. To make large centrifuges capable of running at very
high speeds was a major task undertaken by the Westinghouse
Electric and Manufacturing Company of East Pittsburgh.
9.43. Because of the magnitude of the engineering problems
involved, no large-scale production plant was ever authorized
but a pilot plant was authorized and constructed at Bayway.
It was operated successfully and gave approximately the degree
of separation predicted by theory. This plant was later shut down
and work on the centrifuge method was discontinued. For this
170 The Separation of Isotopes
reason no further discussion of the centrifuge method is given in
this report.
ISOTOPE SEPARATION COMPARED WITH
PLUTONIUM PRODUCTION
9.44. The most important methods of isotope separation that
have been described were known in principle and had been
reduced to practice before the separation of uranium isotopes
became of paramount importance. They had not been applied
to uranium except for the separation of a few micrograms, and
they had not been applied to any substance on a scale comparable
to that now required. But the fundamental questions were of
costs, efficiency, and time, not of principle; in other words, the
problem was fundamentally technical, not scientific. The plu-
tonium production problem did not reach a similar stage until
after the first self-sustaining chain-reacting pile had operated and
the first microgram amounts of plutonium had been separated.
Even after this stage many of the experiments done on the
plutonium project were of vital interest for the military use either
of U-235 or plutonium and for the future development of nuclear
power. As a consequence, the plutonium project has continued
to have a more general interest than the isotope separation proj-
ects. Many special problems arose in the separation projects
which were extremely interesting and required a high order of
scientific ability for their solution but which must still be kept
secret. It is for such reasons that the present non-technical report
has given first emphasis to the plutonium project and will give
less space to the separation projects. This is not to say that the
separation problem was any easier to solve or that its solution
was any less important.
SUMMARY
9.45. Except in electromagnetic separators, isotope separation
depends on small differences in the average behavior of molecules.
Such effects are used in six "statistical" separation methods:
(1) gaseous diffusion, (2) distillation, (3) centrifugation, (4)
The Separation of Isotopes 171
thermal diffusion, (5) exchange reactions, (6) electrolysis.
Probably only (1), (3), and (4) are suitable for uranium; (2),
(5), and (6) are preferred for the separation of deuterium from
hydrogen. In all these "statistical" methods the separation factor
is small so that many stages are required, but in the case of each
method large amounts of material may be handled. All these
methods had been tried with some success before 1940; however,
none had been used on a large scale and none had been used for
uranium. The scale of production by electromagnetic methods
was even smaller but the separation factor was larger. There
were apparent limitations of scale for the electromagnetic
method. There were presumed to be advantages in combining
two or more methods because of the differences in performance
at different stages of separation. The problem of developing any
or all of these separation methods was not a scientific one of
principle but a technical one of scale and cost. These develop-
ments can therefore be reported more briefly than those of the
plutonium project although they are no less important. A pilot
plant was built using centrifuges and operated successfully. No
large-scale plant was built. Plants were built for the production
of heavy water by two different methods.
CHAPTER X. THE SEPARATION OF THE
URANIUM ISOTOPES BY GASEOUS
DIFFUSION
INTRODUCTION
10.1. It was in February 1940 that small amounts of concen-
trated fractions of the three uranium isotopes of masses 234, 235,
and 238 were obtained by A. O. Nier using his mass spectrometer
and were turned over to E. T. Booth, A. von Grosse, and J. R.
Dunning for investigation with the Columbia University cyclo-
tron. These men soon demonstrated that U-235 was the isotope
susceptible to fission by thermal neutrons. It was natural, there-
fore, that this group, under the leadership of Dunning, became
more interested than ever in the large-scale separation of the
uranium isotopes.
10.2. The diffusion method was apparently first seriously
reviewed by Dunning in a memorandum to G. B. Pegram, which
was sent to L. J. Briggs in the fall of 1940. This memorandum
summarized preliminary investigations that had been carried
on by E. T. Booth, A. von Grosse and J. R. Dunning. Work was
accelerated in 1941 with financial help provided by a contract
that H. C. Urey had recived from the Navy for the study of
isotope separation principally by the centrifuge method. During
this period F. G. Slack of Vanderbilt University and W. F.
Libby of the University of California joined the group. An OSRD
contract (OEMsr-106) calling specifically for diffusion studies
went into effect on July 1, 1941, and ran for a year. The work con-
tinued on an expanding scale under a series of OSRD and Army
contracts through the spring of 1945. Up until May 1943 Dun-
ning was in immediate charge of this work; Urey was in charge
of statistical methods in general. From that time until February
1945 Urey was in direct charge of the Columbia part of the
172
Diffusion Separation 173
diffusion work, with Dunning continuing as director of one of
the principal divisions. On March 1, 1945, the laboratory was
taken over from Columbia by Carbide and Carbon Chemicals
Corporation. Early in 1 942, at the suggestion of E. V. Murphree,
the M. W. Kellogg Company was brought in to develop plans
for large-scale production of diffusion-plant equipment and
eventually to build a full-scale plant. To carry out this under-
taking, a new subsidiary company was formed called the Kellex
Corporation. In January 1943, Carbide and Carbon Chemicals
Corporation was given the responsibility for operating the plant.
10.3. As stated in Chapter IV, by the end of 1941 the possi-
bility of separating the uranium hexafluorides had been demon-
strated in principle by means of a single-stage diffusion unit
employing a porous barrier (for example, a barrier made by
etching a thin sheet of silver-zinc alloy with hydrochloric acid) .
A considerable amount of work on barriers and pumps had also
been done but no answer entirely satisfactory for large-scale
operation had been found. Also, K. Cohen had begun a series
of theoretical studies, to which reference has already been made,
as to what might be the best way to use the diffusion process,
i.e., as to how many stages would be required, what aggregate
area of barrier would be needed, what volume of gas would have
to be circulated, etc. Theoretical studies and process develop-
ment by M. Benedict added much to knowledge in this field and
served as the basis of design of the large plant.
10.4. Reports received from the British, and the visit by the
British group in the winter of 1941-1942, clarified a number of
points. At that time the British were planning a diffusion sepa-
ration plant themselves so that the discussions with F. Simon, R.
Peierls, and others were particularly valuable.
THE PRINCIPLES OF SEPARATION BY DIFFUSION
A SINGLE DIFFUSION STAGE
10.5. As was explained in the last chapter, the rate of diffusion
of a gas through an ideal porous barrier is inversely proportional
174 Diffusion Separation
to the square root of its molecular weight. Thus if a gas consisting
of two isotopes starts to diffuse through a barrier into an evacu-
ated vessels, the lighter isotope (of molecular weight MI) diffuses
more rapidly than the heavier (of molecular weight M2). The
result, for a short period of time, at least, is that the relative con-
centration of the lighter isotope is greater on the far side of the
barrier than on the near side. But if the process is allowed to
continue indefinitely, equilibrium will become established and
the concentrations will become identical on both sides of the
barrier. Even if the diffusate gas (the gas which has passed through
the barrier) is drawn away by a pump, the relative amount of the
heavy isotope passing through the barrier will increase since
the light isotope on the near side of the barrier has been depleted
by the earlier part of the diffusion.
10.6. For a single diffusion operation, the increase in the rela-
tive concentration of the light isotope in the diffused gas com-
pared to the feed gas can be expressed in terms of the separation
factor r or the enrichment factor, r 1, both defined in para-
graph 9.8 of the last chapter. A rather simple equation can be
derived which gives r 1 in terms of the molecular weights and
the fraction of the original gas which has diffused. If this fraction
is very small, the equation reduces to r = a, the "ideal separation
factor" of paragraph 9.14. If the fraction diffused is appreciable,
the equation shows the expected diminution in separation. For
example, if half the gas diffuses, r 1 = .69 (a 1), or for
uranium hexafluoride r = 1 .003 compared to the value of 1 .0043
when a very small fraction of the original gas has diffused.
THE CASCADE
10.7. To separate the uranium isotopes, many successive
diffusion stages (i.e., a cascade) must be used since a 1.0043
for U 235 F 6 and U 238 F 6 , a possible gas for uranium separation.
Studies by Cohen and others have shown that the best flow
arrangement for the successive stages is that in which half the
gas pumped into each stage diffuses through the barrier, the
other (impoverished) half being returned to the feed of the next
Diffusion Separation 175
lower stage. For such an arrangement, as we have seen, the ideal
separating effect between the feed and output of a single stage
is 0.69(a 1). This is often called e, the "overall enrichment
per stage." For the uranium hexafluorides, = 0.003, in theory;
but it is somewhat less in practice as a result of "back diffusion,"
of imperfect mixing on the high pressure side, and of imperfec-
tions in the barrier. The first experimental separation of the
uranium hexafluorides (by E. T. Booth, H. C. Paxton, and C. B.
Slade) gave results corresponding to = 0.0014. If one desires
to produce 99 per cent pure U 235 F 6 , and if one uses a cascade
in which each stage has a reasonable overall enrichment factor,
then it turns out that roughly 4,000 stages are required.
GAS CIRCULATION IN THE CASCADE
10.8. Of the gas that passes through the barrier of any given
stage, only half passes through the barrier of the next higher
stage, the other half being returned to an earlier stage. Thus
most of the material that eventually emerges from the cascade
has been recycled many times. Calculation shows that for an
actual uranium-separation plant it may be necessary to force
through the barriers of the first stage 100,000 times the volume
of gas that comes out the top of the cascade (i.e., as desired
product U 235 F 6 ). The corresponding figures for higher stages
fall rapidly because of reduction in amount of unwanted material
(U 238 F 6 ) that is carried along.
THE PROBLEM OF LARGE-SCALE SEPARATION
INTRODUCTION
10.9. By the time of the general reorganization of the atomic-
bomb project in December 1 941 , the theory of isotope separation
by gaseous diffusion was well understood. Consequently it was
possible to define the technical problems that would be en-
countered in building a large-scale separation plant. The decisions
as to scale and location of such plant were not made until the
\vinter of 1942-1943, that is, about the same time as the corre-
176 Diffusion Separation
spending decisions were being made for the plutonium produc-
tion plants.
THE OBJECTIVE
10.10. The general objective of the large-scale gaseous diffusion
plant was the production each day of a specified number of
grams of uranium containing of the order of ten times as much
U-235 as is present in the same quantity of natural uranium.
However, it was apparent that the plant would be rather flexible
in operation, and that considerable variations might be made in
the degree of enrichment and yield of the final product.
THE PROCESS GAS
10.11. Uranium hexafluoride has been mentioned as a gas
that might be suitable for use in the plant as "process gas"; not
the least of its advantages is that fluorine has only one isotope so
that the UFe molecules of any given uranium isotope all have the
same mass. This gas is highly reactive and is actually a solid at
room temperature and atmospheric pressure. Therefore the study
of other gaseous compounds of uranium was urgently undertaken.
As insurance against failure in this search for alternative gases,
it was necessary to continue work on uranium hexafluoride, as
in devising methods for producing and circulating the gas.
THE NUMBER OF STAGES
10.12. The number of stages required in the main cascade of
the plant depended only on the degree of enrichment desired
and the value of overall enrichment per stage attainable with
actual barriers. Estimates were made which called for several
thousand stages. There was also to be a "stripping" cascade of
several hundred stages, the exact number depending on how
much unseparated U-235 could economically be allowed to go
to waste.
BARRIER AREA
10.13. We have seen that the total value of gas that must
diffuse through the barriers is very large compared to the volume
Diffusion Separation 177
of the final product. The rate at which the gas diffuses through
unit area of barrier depends on the pressure difference on the
two sides of the barrier and on the porosity of the barrier. Even
assuming full atmospheric pressure on one side and zero pressure
on the other side, and using an optimistic figure for the porosity,
calculations showed that many acres of barrier would be needed
in the large-scale plant.
BARRIER DESIGN
10.14. At atmospheric pressure the mean free path of a mole-
cule is of the order of a ten-thousandth of a millimeter or one
tenth of a micron. To insure true "diffusive" flow of the gas, the
diameter of the myriad holes in the barrier must be less than one
tenth the mean free path. Therefore the barrier material must
have almost no holes which are appreciably larger than 0.01
micron (4 X 10~ 7 inch), but must have billions of holes of this
size or smaller. These holes must not enlarge or plug up as the
result of direct corrosion or dust coming from corrosion elsewhere
in the system. The barrier must be able to withstand a pressure
"head" of one atmosphere. It must be amenable to manufacture
in large quantities and with uniform quality. By January 1 942, a
number of different barriers had been made on a small scale and
tested for separation factor and porosity. Some were thought to
be very promising, but none had been adequately tested for actual
large-scale production and plant use.
PUMPING AND POWER REQUIREMENTS
10.15. In any given stage approximately half of the material
entering the stage passes through the barrier and on to the next
higher stage, while the other half passes back to the next lower
stage. The diffused half is at low pressure and must be pumped to
high pressure before feeding into the next stage. Even the undif-
fused portion emerges at somewhat lower pressure than it entered
and cannot be fed back to the lower stage without pumping.
Thus the total quantity of gas per stage (comprising twice the
178 Diffusion Separation
amount which flows through the barrier) has to be circulated by
means of pumps.
10.16. Since the flow of gas through a stage varies greatly with
the position of the stage in the cascade, the pumps also vary
greatly in size or number from stage to stage. The type and
capacity of the pump required for a given stage depends not
only on the weight of gas to be moved but on the pressure rise
required. Calculations made at this time assumed a fore pressure
of one atmosphere and a back pressure (i.e., on the low pressure
side of the barrier) of one tenth of an atmosphere. It was esti-
mated that thousands of pumps would be needed and that
thousands of kilowatts would be required for their operation.
Since an unavoidable concomitant of pumping gas is heating
it, it was evident that a large cooling system would have to be
provided. By early 1942, a good deal of preliminary work had
been done on pumps. Centrifugal pumps looked attractive in
spite of the problem of sealing their shafts, but further experi-
mental work was planned on completely sealed pumps of various
types.
LEAKS AND CORROSION
10.17. It was clear that the whole circulating system com-
prising pumps, barriers, piping, and valves would have to be
vacuum tight. If any lubricant or sealing medium is needed in
the pumps, it should not react with the process gas. In fact none
of the materials in the system should react with the process gas
since such corrosion would lead not only to plugging of the
barriers and various mechanical failures but also to absorption
(i.e., virtual disappearance) of uranium which had already been
partially enriched.
ACTUAL vs. IDEAL CASCADE
10.18. In an ideal cascade, the pumping requirements change
from stage to stage. In practice it is not economical to provide a
different type of pump for every stage. It is necessary to deter-
mine how great a departure from the ideal cascade (i.e., what
Diffusion Separation 179
minimum number of pump types) should be employed in the
interest of economy of design, repair, etc. Similar compromises
are used for other components of the cascade.
HOLD-UP AND START-UP TIME
10.19. When first started, the plant must be allowed to run
undisturbed for some time, until enough separation has been
effected so that each stage contains gas of appropriate enrich-
ment. Only after such stabilization is attained is it desirable to
draw off from the top stage any of the desired product. Both
the amount of material involved (the hold-up) and the time
required (the start-up time) are great enough to constitute major
problems in their own right.
EFFICIENCY
10.20. It was apparent that there would be only three types of
material loss in the plant contemplated, namely: loss by leakage,
loss by corrosion (i.e., chemical combination and deposition),
and loss in plant waste. It was expected that leakage could be
kept very small and that after an initial period of operation
loss from corrosion would be small. The percentage of material
lost in plant waste would depend on the number of stripping
stages.
DETAILED DESIGN
10.21. Questions as to how the barrier material was to be used
(whether in tubes or sheets, in large units or small units), how
mixing was to be effected, and what controls and instruments
would be required were still to be decided. There was little
reason to expect them to be unanswerable, but there was no
doubt that they would require both theoretical and experimental
study.
SUMMARY OF THE PROBLEM
10.22. By 1942 the theory of isotope separation by gaseous
diffusion had been well worked out, and it became clear that a
180 Diffusion Separation
very large plant would be required. The major equipment items
in this plant were diffusion barriers and pumps. Neither the
barriers nor the pumps which were available at that time had
been proved generally adequate. Therefore the further develop-
ment of pumps and barriers was especially urgent. There were
also other technical problems to be solved, these involving cor-
rosion, vacuum seals, and instrumentation.
ORGANIZATION
10.23. As we mentioned at the beginning of this chapter, the
diffusion work was initiated by J. R. Dunning. The work was
carried on under OSRD auspices at Columbia University until
May 1, 1943, when it was taken over by the Manhattan District.
In the summer of 1943 the difficulties encountered in solving
certain phases of the project led to a considerable expansion,
particularly of the chemical group. H. C. Urey, then director
of the work, appointed H. S. Taylor of Princeton associate
director and added E. Mack, Jr. of Ohio State, G. M. Murphy
of Yale, and P. H. Emmett of Johns Hopkins to the senior staff.
Most of the work was moved out of the Columbia laboratories
to a large building situated near by. The chemists at Princeton
who had been engaged in heavy water studies were assigned some
of the barrier research problems. Early in 1944, L. M. Currie
of the National Carbon Company became another associate
director to help Urey in his liaison and administrative work.
10.24. As has been mentioned, the M. W. Kellogg Company
was chosen early in 1942 to plan the large scale plant. For this
purpose Kellogg created a special subsidiary called The Kellex
Corporation, with P. C. Leith as executive in charge and tech-
nical head and, responsible to him, A. L. Baker as Project Man-
ager, and J. H. Arnold as Director of Research and Development.
The new subsidiary carried on research and development in its
Jersey City laboratories and in the laboratory building referred
to in the paragraph above; developed the process and engineering
designs; and procured materials for the large-scale plant and
supervised its construction. The plant was constructed by the
Diffusion Separation 181
J. A. Jones Construction Company, Incorporated, of Charlotte,
North Carolina.
10.24-a. The Kellex Corporation, unlike conventional indus-
trial firms, was a cooperative of scientists, engineers and adminis-
trators recruited from essentially all branches of industry and
gathered for the express purpose of carrying forward this one job.
Service was on a voluntary basis, individuals prominent in indus-
try freely relinquishing their normal duties and responsibilities
to devote full time to Kellex activities. As their respective tasks
are being completed these men are returning to their former
positions in industry.
10.25. In January 1943, Carbide and Carbon Chemicals
Corporation were chosen to be the operators of the completed
plant. Their engineers soon began to play a large role not only
in the planning and construction but also in the research work.
RESEARCH, DEVELOPMENT, CONSTRUCTION, AND
PRODUCTION, 1942 TO 1945
PRODUCTION OF BARRIERS
10.26. Even before 1942, barriers had been developed that
were thought to be satisfactory. However, the barriers first
developed by E. T. Booth, H. C. Paxton, and C. B. Slade were
never used on a large scale because of low mechanical strength
and poor corrosion resistance. In 1942, under the general super-
vision of Booth and F. G. Slack and with the cooperation of
various scientists including F. C. Nix of the Bell Telephone
Laboratories, barriers of a different type were produced. At
one time, a barrier developed by E. O. Norris and E. Adler
was thought sufficiently satisfactory to be specified for plant use.
Other barriers were developed by combining the ideas of
several men at the Columbia laboratories (by now christened the
SAM Laboratories), Kellex, Bell Telephone Laboratories, Bake-
lite Corporation, Houdaille-Hershey Corporation, and others.
The first specimens of the type of barrier selected for general use
in the plant were prepared by C. A. Johnson of Kellex, and the
182 Diffusion Separation
barrier was perfected under the general supervision of H. S.
Taylor. One modification of this barrier developed by the SAM
Laboratories represented a marked improvement in quality and
is being used in a large number of stages of the plant. By 1945 the
problem was no longer one of barely meeting minimum specifica-
tions, but of making improvements resulting in greater rate of
output or greater economy of operation.
10.27. Altogether the history of barrier development reminds
the writer of the history of the "canning" problem of the plu-
tonium project. In each case the methods were largely cut and
try, and satisfactory or nearly satisfactory solutions were repeat-
edly announced; but in each case a really satisfactory solution
was not found until the last minute and then proved to be far
better than had been hoped.
PUMPS AND SEALS
10.28. The early work on pumps was largely under the super-
vision of H. A. Boorse of Columbia University. When Kellex came
into the picture in 1942, its engineers, notably G. W. Watts,
J. S. Swearingen and O. C. Brewster, took leading positions in
the development of pumps and seals. It must be remembered
that these pumps are to be operated under reduced pressure, must
not leak, must not corrode, and must have as small a volume as
possible. Many different types of centrifugal blower pumps and
reciprocating pumps were tried. In one of the pumps for the
larger stages, the impeller is driven through a coupling contain-
ing a very novel and ingenious type of seal. Another type of pump
is completely enclosed, its centrifugal impeller and rotor being
run from outside, by induction.
MISCELLANEOUS DEVELOPMENTS
10.29. As in the plutonium problem, so here also, there were
many questions of corrosion, etc., to be investigated. New coolants
and lubricants were developed by A. L. Henne and his associ-
ates, by G. H. Cady, by W T . T. Miller and his co-workers, by
E. T. McBee and his associates, and by scientists of various
Diffusion Separation 183
corporations including Hooker Electrochemical Co., the du Pont
Co. and the Harshaw Chemical Co. The research and develop-
ment and plant requirements for these materials and other special
chemicals were coordinated by R. Rosen, first under OSRD and
later for Kellex. Methods of pretreating surfaces against corrosion
were worked out. Among the various instruments designed or
adapted for project use, the mass spectrograph deserves special
mention. The project was fortunate in having the assistance of
A. O. Nier of the University of Minnesota and later of Kellex
whose mass spectrograph methods of isotope analysis were suffi-
ciently advanced to become of great value to the project, as in
analyzing samples of enriched uranium. Mass spectrographs
were also used in pretesting parts for vacuum leaks and for
detecting impurities in the process gas in the plant.
PILOT PLANTS
10.30. Strictly speaking, there was no pilot plant. That is to
say, there was no small-scale separation system set up using the
identical types of blowers, barriers, barrier mountings, cooling,
etc., that were put into the main plant. Such a system could
not be set up because the various elements of the plant were not
all available prior to the construction of the plant itself. To pro-
ceed with the construction of the full-scale plant under these
circumstances required foresight and boldness.
10.31. There was, however, a whole series of so-called pilot
plants which served to test various components or groups of
components of the final plant. Pilot plant No. 1 was a 12-stage
plant using a type of barrier rather like that used in the large-
scale plant, but the barrier material was not fabricated in the
form specified for the plant and the pumps used were sylphon-
sealed reciprocating pumps, not centrifugal pumps. Work on this
plant in 1943 tested not only the barriers and general system of
separation but gave information about control valves, pressure
gauges, piping, etc. Pilot plant No. 2, a larger edition of No. 1
but with only six stages, was used in late 1943 and early 1944,
particularly as a testing unit for instruments. Pilot plant No. 3a,
184 Diffusion Separation
using centrifugal blowers and dummy diffusers, was also intended
chiefly for testing instruments. Pilot plant No. 3b was a real
pilot plant for one particular section of the large-scale plant. Pilot
plants using full-scale equipment at the plant site demonstrated
the vacuum tightness, corrosion resistance and general operability
of the equipment.
PLANT AUTHORIZATION
10.32. In December 1942, the Kellogg Company was author-
ized to proceed with preliminary plant design and in January
1943 the construction of a plant was authorized.
THE SITE
10.33. As stated in an earlier chapter, a site in the Tennessee
Valley had originally been chosen for all the Manhattan District
plants, but the plutonium plant was actually constructed else-
where. There remained the plutonium pilot plant already
described, the gaseous diffusion plant, the electromagnetic
separation plant (see Chapter XI), and later the thermal dif-
fusion plant which were all built in the Tennessee Valley at the
Clinton site, known officially as the Clinton Engineer Works.
10.34. This site was examined by Colonel Marshall, Colonel
Nichols, and representatives of Stone and Webster Engineering
Corporation in July 1942, and its acquisition was recommended.
This recommendation was endorsed by the OSRD S-l Executive
Committee at a meeting in July 1942. Final approval was given
by Major General L. R. Groves after personal inspection of the
70-square-mile site. In September 1942, the first steps were taken
to acquire the tract, which is on the Clinch River about thirty
miles from Knoxville, Tennessee, and eventually considerably
exceeded 70 square miles. The plutonium pilot plant is located
in one valley, the electromagnetic separation plant in an adjoin-
ing one, and the diffusion separation plant in a third.
10.35. Although the plant and site development at Hanford
is very impressive, it is all under one company dealing with but
one general operation so that it is in some respects less interesting
Diffusion Separation 185
than Clinton, which has a great multiplicity of activity. To
describe the Clinton site, with its great array of new plants, its
new residential districts, new theatres, new school system, seas
of mud, clouds of dust, and general turmoil is outside the scope
of this report.
DATES OF START OF CONSTRUCTION
10.36. Construction of the steam power plant for the diffusion
plant began on June 1, 1943. It is one of the largest such power
plants ever built. Construction of other major buildings and
plants started between August 29, 1943 and September 10, 1943.
OPERATION
10.37. Unlike Hanford, the diffusion plant consists of so many
more or less independent units that it was put into operation
section by section, as permitted by progress in constructing and
testing. Thus there was no dramatic start-up date nor any unto-
ward incident to mark it. The plant was in successful operation
before the summer of 1945.
10.38. For the men working on gaseous diffusion it was a long
pull from 1940 to 1945, not lightened by such exciting half-way
marks as the first chain-reacting pile at Chicago. Perhaps more
than any other group in the project, those who have worked on
gaseous diffusion deserve credit for courage and persistence as
well as scientific and technical ability. For security reasons, we
have not been able to tell how they solved their problems even
in many cases found several solutions, as insurance against
failure in the plant. It has been a notable achievement. In these
five years there have been periods of discouragement and pessi-
mism. They are largely forgotten now that the plant is not only
operating but operating consistently, reliably, and with a per-
formance better than had been anticipated.
SUMMARY
10.39. Work at Columbia University on the separation of
isotopes by gaseous diffusion began in 1940, and by the end of
186 Diffusion Separation
1942 the problems of large-scale separation of uranium by this
method had been well defined. Since the amount of separation
that could be effected by a single stage was very small, several
thousand successive stages were required. It was found that
the best method of connecting the many stages required extensive
recycling so that thousands of times as much material would pass
through the barriers of the lower stages as would ultimately
appear as product from the highest stage.
10.40. The principal problems were the development of satis-
factory barriers and pumps. Acres of barrier and thousands of
pumps were required. The obvious process gas was uranium hexa-
fluoride for which the production and handling difficulties were
so great that a search for an alternative was undertaken. Since
much of the separation was to be carried out at low pressure,
problems of vacuum technique arose, and on a previously
unheard-of scale. Many problems of instrumentation and control
were solved; extensive use was made of various forms of mass
spectrograph.
10.41. The research was carried out principally at Columbia
under Dunning and Urey. In 1942, the M. W. Kellogg Company
was chosen to develop the process and equipment and to design
the plant and set up the Kellex Corporation for the purpose. The
plant was built by the J. A. Jones Construction Company. The
Carbide and Carbon Chemicals Corporation was selected as
operating company.
10.42. A very satisfactory barrier was developed although the
final choice of barrier type was not made until the construction
of the plant was well under way at Clinton Engineer Works in
Tennessee. Two types of centrifugal blower were developed to
the point where they could take care of the pumping require-
ments. The plant was put into successful operation before the
summer of 1945.
CHAPTER XL ELECTROMAGNETIC
SEPARATION OF URANIUM
ISOTOPES
INTRODUCTION
11.1. In Chapter IV we said that the possibility of large-scale
separation of the uranium isotopes by electromagnetic means was
suggested in the fall of 1941 by E. O. Lawrence of the University
of California and H. D. Smyth of Princeton University. In Chap-
ter IX we described the principles of one method of electromag-
netic separation and listed the three limitations of that method:
difficulty of producing ions, limited fraction of ions actually used,
and space charge effects.
11.2. By the end of December 1941, when the reorganization
of the whole uranium project was effected, Lawrence had already
obtained some samples of separated isotopes of uranium and in
the reorganization he was officially placed in charge of the prepa-
ration of further samples and the making of various associated
physical measurements. However, just as the Metallurgical
Laboratory very soon shifted its objective from the physics of
the chain reaction to the large-scale production of plutonium,
the objective of Lawrence's division immediately shifted to the
effecting of large-scale separation of uranium isotopes by electro-
magnetic methods. This change was prompted by the success
of the initial experiments at California and by the development
at California and at Princeton of ideas on other possible methods.
Of the many electromagnetic schemes suggested, three soon were
recognized as being the most promising: the "calutron" mass
separator, the magnetron-type separator later developed into
the "ionic centrifuge," and the "isotron" method of ''bunching"
a beam of ions. The first two of these approaches were followed
at California and the third at Princeton. After the first few
187
188 Electromagnetic Separation
months, by far the greatest effort was put on the calutron, but
some work on the ionic centrifuge was continued at California
during the summer of 1942 and was further continued by J.
Slepian at the Westinghouse laboratories in Pittsburgh on a
small scale through the winter of 1944-1945. Work on the isotron
was continued at Princeton until February 1943, when most of
the group was transferred to other work. Most of this chapter
will be devoted to the calutron since that is the method that has
resulted in large-scale production of U-235. A brief description
will also be given of the thermal diffusion plant built to provide
enriched feed material for the electromagnetic plant.
11.3. Security requirements make it impossible here as for
other parts of the project to present many of the most interesting
technical details. The importance of the development is con-
siderably greater than is indicated by the amount of space which
is given it here.
ELECTROMAGNETIC MASS SEPARATORS
PRELIMINARY WORK
11.4. A. O. Nier's mass spectrograph was set up primarily to
measure relative abundances of isotopes, not to separate large
samples. Using vapor from uranium bromide Nier had prepared
several small samples of separated isotopes of uranium, but his
rate of production was very low indeed, since his ion current
amounted to less than one micro-ampere. (A mass spectrograph
in which one micro-ampere of normal uranium ions passes
through the separating fields to the collectors will collect about
one microgram of U-235 per 16-hour day.) The great need of
samples of enriched U-235 for nuclear study was recognized
early by Lawrence, who decided to see what could be done with
the help of the 37-inch (cyclotron) magnet at Berkeley. The
initial stages of this work were assisted by a grant from the Re-
search Corporation of New York, which was later repaid. Be-
ginning January 1, 1942, the entire support came from the OSRD
through the S-l Committee. Later, as in other parts of the
Electromagnetic Separation 189
uranium project, the contracts were taken over by the Manhattan
District.
11.5. At Berkeley, after some weeks of planning, the 37-inch
cyclotron was dismantled on November 24, 1941, and its magnet
was used to produce the magnetic field required in what came
to be called a "calutron" (a name representing a contraction of
"California University cyclotron"). An ion source consisting of
an electron beam traversing the vapor of a uranium salt was set
up corresponding to the ion source shown in the drawing in
Chap. IX, p. 164. Ions were then accelerated to the slit 82 through
which they passed into the separating region where the magnetic
field bent their paths into semicircles terminating at the collector
slit. By December 1, 1941, molecular ion beams from the residual
gas were obtained, and shortly thereafter the beam consisting
of singly charged uranium ions (U+) was brought up to an
appreciable strength. It was found that a considerable proportion
of the ions leaving the source were U+ ions. For the purpose of
testing the collection of separated samples, a collector with two
pockets was installed, the two pockets being separated by a
distance appropriate to the mass numbers 235 and 238. Two
small collection runs using U+ beams of low strength were made
in December, but subsequent analyses of the samples showed
only a small separation factor. By the middle of January 1942,
a run had been made with a reasonable beam strength and an
aggregate flow or through-put of appreciable amount which
showed a much improved separation factor. By early February
1 942, beams of much greater strength were obtained, and Law-
rence reported that good separation factors were obtainable with
such beams. By early March 1942, the ion current had been
raised still further. These results tended to bear out Lawrence's
hopes that space charge could be neutralized by ionization of the
residual gas in the magnet chamber.
INITIATION OF A LARGE PROGRAM
11.6. By this time it was clear that the calutron was potentially
able to effect much larger scale separations than had ever before
190 Electromagnetic Separation
been approached by an electromagnetic method. It was evi-
dently desirable to explore the whole field of electromagnetic
separation. With this end in view, Lawrence mobilized his group
at the Radiation Laboratory of the University of California at
Berkeley and began to call in others to help. Among those
initially at Berkeley were D. Cooksey, P. C. Aebersold, W. M.
Brobeck, F. A. Jenkins, K. R. MacKenzie, W. B. Reynolds,
D. H. Sloan, F. Oppenheimer, J. G. Backus, B. Peters, A. C.
Helmholz, T. Finkelstein, and W. E. Parkins, Jr. Lawrence
called back some of his former students, including R. L. Thornton,
J. R. Richardson, and others. Among those working at Berkeley
for various periods were L. P. Smith from Cornell, E. U. Condon
and J. Slepian from Westinghouse, and I. Langmuir and
K. H. Kingdon from General Electric. During this early period
J. R. Oppenheimer was still at Berkeley and contributed some
important ideas. In the fall of 1 943 the group was further strength-
ened by the arrival of a number of English physicists under the
leadership of M. L. Oliphant of the University of Birmingham.
11.7. Initially a large number of different methods were con-
sidered and many exploratory experiments were performed. The
main effort, however, soon became directed towards the develop-
ment of the calutron, the objective being a high separation factor
and a large current in the positive ion beam.
IMMEDIATE OBJECTIVES
11.8. Of the three apparent limitations listed in the first
paragraph difficulty of producing ions, limited fraction of ions
actually used, and space charge effects only the last had yielded
to the preliminary attack. Apparently space charge in the neigh-
borhood of the positive ion beam could be nullified to a very
great extent. There remained as the immediate objectives a more
productive ion source and more complete utilization of the ions.
11.9. The factors that control the effectiveness of an ion source
are many. Both the design of the source proper and the method
of drawing ions from it are involved. The problems to be solved
cannot be formulated simply and must be attacked by methods
Electromagnetic Separation 191
that are largely empirical. Even if security restrictions permitted
an exposition of the innumerable forms of ion source and acceler-
ating system that were tried, such exposition would be too
technical to present here.
11.10. Turning to the problem of effecting more complete
utilization of the ions, we must consider in some detail the princi-
ple of operation of the calutron. The calutron depends on the
fact that singly charged ions moving in a uniform magnetic
field perpendicular to their direction of motion are bent into
circular paths of radius proportional to their momenta. Con-
sidering now just a single isotope, it is apparent that the ions
passing through the two slits (and thus passing into the large
evacuated region in which the magnetic field is present) do not
initially follow a single direction, but have many initial directions
lying within a small angle, whose size depends on the width of
the slits. Fortunately, however, since all the ions of the isotope
in question follow curved paths of the same diameter, ions starting
out in slightly different directions tend to meet again or almost
meet again after completing a semicircle. It is, of course, at
this position of reconvergence that the collector is placed.
Naturally, the ions of another isotope (for example, ions of mass
238 instead of 235) behave similarly, except that they follow
circles of slightly different diameter. Samples of the two isotopes
are caught in collectors at the two different positions of recon-
vergence. Now the utilization of a greater fraction of the ions
originally produced may be accomplished readily enough by
widening the two slits referred to. But to widen the slits to any
great extent without sacrificing sharpness of focus at the recon-
192 Electromagnetic Separation
vergence positions is not easy. Indeed it can be accomplished
only by use of carefully proportioned space variations in the
magnetic field strength. Fortunately, such variations were worked
out successfully.
11.11. Another problem, not so immediate but nevertheless
recognized as important to any production plant, was that of
more efficient use of the magnetic field. Since large electro-
magnets are expensive both to build and to operate, it was
natural to consider using the same magnetic field for several ion
beams. The experimental realization of such an economical
scheme became a major task of the laboratory.
THE GIANT MAGNET
11.12. Although the scale of separation reached by March 1 942
was much greater than anything that had previously been done
with an electromagnetic mass separator, it was still very far from
that required to produce amounts of material that would be of
military significance. The problems that have been outlined not
only had to be solved, but they had to be solved on a grand scale.
The 37-inch cyclotron magnet that had been used was still
capable of furnishing useful information, but larger equipment
was desirable. Fortunately a very much larger magnet, intended
for a giant cyclotron, had been under construction at Berkeley.
This magnet, with a pole diameter of 184 inches and a pole gap
of 72 inches, was to be the largest in existence. Work on it had
been interrupted because of the war, but it was already suffi-
ciently advanced so that it could be finished within a few months
if adequate priorities were granted. Aside from the magnet itself,
the associated building, laboratories, shops, etc., were almost
ideal for the development of the calutron. Needless to say, work
was resumed on the giant magnet and by the end of May 1942,
it was ready for use. *
* The construction of the giant cyclotron had been undertaken with
private funds largely supplied by the Rockefeller Foundation, augmented
by donations from the Research Corporation, the John and Mary Markel
Foundation, and the University of California. In order to push the construe-
Electromagnetic Separation 193
DEVELOPMENT UP TO SEPTEMBER 1942
11.13. The first experiments using the 37-inch magnet have
been described in a previous paragraph. Later developments
proceeded principally along these two lines: construction and
installation of a properly engineered separation unit for the
37-inch magnet, and design and construction of experimental
separation units to go into the big magnet.
11.14. Besides the gradual increase in ion beam strength and
separation factor that resulted from a series of developments in
the ion source and in the accelerating system, the hoped-for
improvement in utilization of ions was achieved during the
summer of 1 942, using the giant magnet. Further, it was possible
to maintain more than one ion beam in the same magnetic
separating region. Experiments on this latter problem did run
into some difficulties, however, and it appeared that there might
be limitations on the number of sources and receivers that could
be put in a single unit as well as on the current that could be
used hi each beam without spoiling the separation.
11.15. It was evident that many separator units would be
needed to get an amount of production of military significance.
Therefore, consideration was given to various systems of com-
bining groups of units in economical arrangements. A scheme
was worked out which was later used in the production plants
and which has proved satisfactory.
ADVANTAGES OF THE ELECTROMAGNETIC SYSTEM
11.16. In September 1942, both the gaseous diffusion and the
centrifugal methods of uranium isotope separation had been
under intensive study and for a longer period than in the case
of the electromagnetic 'method. Both of these methods gaseous
diffusion and centrifuge looked feasible for large-scale pro-
duction of U-235, but both would require hundreds of stages to
achieve large-scale separation. Neither had actually produced
tion as fast as possible overtime work was required at additional expense.
To cover these costs the Rockefeller Foundation made an extra appropriation.
194 Electromagnetic Separation
any appreciable amounts of separated U-235. No large-scale
plant for plutonium production was under way, and the self-
sustaining chain reaction which was to produce plutonium had
not yet been proved attainable. But in the case of the electro-
magnetic method, after the successful separation of milligram
amounts, there was no question as to the scientific feasibility.
If one unit could separate 10 mg a day, 100,000,000 units could
separate one ton a day. The questions were of cost and time.
Each unit was to be a complicated electromagnetic device re-
quiring high vacuum, high voltages, and intense magnetic fields;
and a great deal of research and development work would be
required before complete, large-scale, units could be constructed.
Many skilled operators would probably be needed. Altogether, at
that time it looked very expensive, but it also looked certain and
relatively quick. Moreover, the smallness of the units had the
advantage that development could continue, modifications could
be made in the course of construction or, within limits, after
construction, and Capacity could always be expanded by building
new units.
POLICY QUESTION
11.17. On the basis of rather incomplete scientific and engi-
neering information on all the methods and on the basis of
equally dubious cost estimates, decisions had to be made on three
issues: (1) whether to build an electromagnetic plant; (2) how
big such a plant should be; (3) at what point of development the
design should be frozen.
APPROVAL OF PLANT CONSTRUCTION
11.18. On the strength of the results reported on experiments
at Berkeley in the summer of 1942, the S-l Executive Committee,
at a meeting at Berkeley on September 1 3-14, 1 942, recommended
that commitments be made by the Army for an electromagnetic
separation plant to be built at the Tennessee Valley site (Clinton
Engineer Works). It was recommended that it should be agreed
that commitments for this plant might be cancelled on the basis
Electromagnetic Separation 195
of later information. It was recommended that a pilot plant
should be erected at the Tennessee Valley site as soon as possible.
(However, this recommendation was subsequently withdrawn and
such a pilot plant was never built.) The construction of a pro-
duction plant was authorized by General Groves on November 5,
1942, with the understanding that the design for the first units
was to be frozen immediately.
ORGANIZATION FOR PLANNING AND CONSTRUCTION
11.19. In describing the production of plutonium, we discussed
the division of responsibility between the Metallurgical Project
and the du Pont Company. The electromagnetic separation plant
was planned and built under a somewhat different scheme of
organization. The responsibility was divided between six major
groups. The Radiation Laboratory at the University of California
was responsible for research and development; the Westinghouse
Electric and Manufacturing Company for making the mechanical
parts, i.e., sources, receivers, pumps, tanks, etc.; the General
Electric Company for the electrical equipment and controls;
the Allis-Chalmers Company for the magnets; the Stone and
Webster Engineering Company for the construction and assembly;
and the Tennessee Eastman Company for operation. All five
industrial concerns kept groups of their engineers at Berkeley
so that a system of frequent informal conference and cross-
checking was achieved. Thus the major part of the planning was
done cooperatively in a single group, even though the details
might be left to the home offices of the various companies.
THE BASIS OF THE TECHNICAL DECISIONS
11.20. Strangely enough, although the theory of the self-
sustaining chain-reacting pile is already well worked out, the
theory of gaseous discharge, after fifty years of intensive study,
is still inadequate for the prediction of the exact behavior of the
ions in a calutron. The amount of U-235 collected per day, and
the purity of the material collected, are affected by many factors,
including: (1) the width, spacing, and shape of the collector,
196 Electromagnetic Separation
(2) the pressure in the .magnet space, (3) the strength and uni-
formity of the magnetic field, (4) the shape and spacing of the
defining slits and accelerating system, (5) the accelerating voltage,
(6) the size and shape of the slit in the arc source from which
the ions come, (7) the current in the arc, (8) the position of the
arc within the arc chamber, (9) the pressure of vapor in the arc
chamber, (10) the chemical nature of the vapor. Evidently there
was not time for a systematic study of all possible combinations of
variables. The development had to be largely intuitive. A variety
of conditions had to be studied and a number of partial interpre-
tations had to be made. Then the accumulated experience of the
group, the "feel" of the problem, had to be translated into specific
plans and recommendations.
TECHNICAL DECISIONS REQUIRED
11.21. (a) The Number of Stages. As in all methods, a compro-
mise must be made between yield and separation factor. In the
electromagnetic system, the separation factor is much higher
than in other systems so that the number of stages required is
small. There was a possibility that a single stage might be suffi-
cient. Early studies indicated that attempts to push the separation
factor so high as to make single-stage operation feasible cut the
yield to an impractically small figure.
11.22. (b) Specifications. The information and experience that
had been acquired on the variables such as those mentioned
above had to be translated into decisions on the following princi-
pal points before design could actually begin: (1) the size of a
unit as determined by the radius of curvature of the ion path,
the length of the source slit, and the arrangement of sources
and receivers; (2) the maximum intensity of magnetic field re-
quired; (3) whether or not to use large divergence of ion beams;
(4) the number of ion sources and receivers per unit; (5) whether
the source should be at high potential or at ground potential;
(6) the number of accelerating electrodes and the maximum
potentials to be applied to them; (7) the power requirements for
arcs, accelerating voltages, pumps, etc.; (8) pumping require-
Electromagnetic Separation 197
ments; (9) number of units per pole gap; (10) number of units
per building.
EXPERIMENTAL UNITS AT BERKELEY
11.23. Most of the design features for the first plant had to be
frozen in the fall of 1942 on the basis of results obtained with
runs made using the giant magnet at Berkeley. The plant design,
however, called for units of a somewhat different type. While
there was no reason to suppose that these changes would introduce
any difference in performance, it was obviously desirable to build
a prototype unit at Berkeley. The construction of this unit was
approved at about the same time that the first plant units were
ordered so that experience with it had no influence on funda-
mental design, but it was finished and operating by April 1943,
that is, six months before the first plant unit. Consequently, it
was invaluable for testing and training purposes. Later, a third
magnet was built in the big magnet building at Berkeley. All
told, there have been six separator units available simultaneously
for experimental or pilot plant purposes at Berkeley. Much
auxiliary work has also been done outside the complete units.
THE ISOTRON SEPARATOR
11.24. As we have already said, H. D. Smyth of Princeton
became interested in electromagnetic methods of separation in
the late summer and fall of 1941. He was particularly interested
in devising some method of using an extended ion source and
beam instead of one limited essentially to one dimension by a
slit system as in the calutron mass separator. A method of actually
achieving separation using an extended ion source was suggested
by R. R. Wilson of Princeton. The device which resulted from
Wilson's ideas was given the deliberately meaningless name
"isotron."
11.25. The isotron is an electromagnetic mass separator using
an extended source of ions, in contrast to the slit sources used in
ordinary mass spectrographs. The ions from the extended source
are first accelerated by a constant, high-intensity, electric field
198 Electromagnetic Separation
and are then further accelerated by a low-intensity electric field
varying at radio frequency and in "saw tooth" manner. The
effect of the constant electric field is to project a strong beam of
ions down a tube with uniform kinetic energy and therefore
with velocities inversely proportional to the square root of the
masses of ions. The varying electric field, on the other hand,
introduces small, periodic variations in ion velocity, and has the
effect of causing the ions to "bunch" at a certain distance down
the tube. (This same principle is used in the klystron high-
frequency oscillator, where the electrons are "bunched" or
"velocity-modulated.") The bunches of ions of different mass
travel with different velocities and therefore become separated.
At the position (actually on area perpendicular to the beam)
where this occurs, an analyzer applies a transverse focussing
electric field with a radio frequency component synchronized
with the arrival of the bunches. The synchronization is such that
the varying component of the transverse field strength is zero
when the U-235 ion bunches come through and a maximum
when the U-238 ion bunches come through. The U-235 beams
are focussed on a collector, but the U-238 bunches are deflected.
Thus the separation is accomplished.
11.26. This scheme was described at the December 18, 1941
meeting of the Uranium Committee and immediately thereafter
was discussed more fully with Lawrence, who paid a visit to
Princeton. The promise of the method seemed sufficient to justify
experimental work, which was begun immediately under an
OSRD contract and continued until February 1943. Since the
idea involved was a novel one, there were two outstanding issues:
(1) whether the method would work at all; (2) whether it could
be developed for large-scale production promptly enough to
compete with the more orthodox methods already under
development.
11.27. An experimental isotron was constructed and put into
operation by the end of January 1942. Preliminary experiments
at that time indicated that the isotopes of lithium could be
separated by the method. The first successful collection of
Electromagnetic Separation 199
partially separated uranium isotopes was made in the spring
of 1942.
11.28. Unfortunately, progress during the summer and fall of
1942 was not as rapid as had been hoped. Consequently, it was
decided to close down the Princeton project in order to permit
sending the personnel to the site where the atomic-bomb labora-
tory was about to get under way. Before the group left Princeton
a small experimental isotron collected several samples of partly
separated uranium. Thus, the method worked; but its large-scale
applicability was not fully investigated.
THE MAGNETRON AND THE IONIC CENTRIFUGE
11.29. In December 1941, when the whole subject of isotope
separation was under discussion at Berkeley, the magnetron was
suggested as a possible mass separator. In the meantime, Smyth
of Princeton had been in contact with L. P. Smith of Cornell
and had discovered that Smith and his students had done a
considerable amount of work and with evidence of success
on the separation of the isotopes of lithium by just such a method.
This was reported to Lawrence in Washington at one of the
December, 1941, meetings of the Uranium Committee. Lawrence
immediately got in touch with Smith, with the result that Smith
worked on the method at Berkeley from February 1942 to
June 1942. J. Slepian of the Westinghouse Research Laboratory
in East Pittsburgh came to Berkeley in the winter of 1941-1942
at Lawrence's invitation and became interested in a modification
of the magnetron which he called an ionic centrifuge. Slepian
stayed at Berkeley most of the time until the fall of 1942, after
which he returned to East Pittsburgh where he continued the
work.
11.30. No separation of uranium was actually attempted in
the magnetron. Experiments with lithium with low ion currents
showed some separation, but no consistent results were obtained
with high ion currents. In the case of the ionic centrifuge, uranium
samples have been collected showing appreciable separation, but
the results have not been clear-cut or consistent.
200 Electromagnetic Separation
THE SITUATION AS OF EARLY 1943
11.31. With the virtual elimination of the isotron and the
ionic centrifuge from the development program, the calutron
separator became the only electromagnetic method worked on
intensively. Construction of initial units of a plant had been
authorized and designs had been frozen for such units, but the
whole electromagnetic program had been in existence for only
a little more than a year and it was obvious that available
designs were based on shrewd guesses rather than on adequate
research. A similar situation might have occurred with the chain-
reacting pile if unlimited amounts of uranium and graphite had
been available before the theory had been worked out or before
the nuclear constants had been well determined. Fortunately the
nature of the two projects was very different, making it a less
speculative venture to build an electromagnetic plant unit
hastily than would have been the case for the pile. Further re-
search and development could proceed advantageously even
while initial units of the plant were being built and operated.
CONSTRUCTION AND OPERATION; MARCH 1943
TO JUNE 1945
COMPARISON WITH DIFFUSION AND PLUTONIUM PLANTS
11.32. The preceding chapters show that the end of 1942 was
a time of decision throughout the uranium project. For it was at
that time that a self-sustaining chain reaction was first produced,
that construction was authorized for the Hanford plutonium
plant, the diffusion plant at Clinton, and the electromagnetic
plant at Clinton. The diffusion plant was more flexible than the
plutonium plant, since the diffusion plant could be broken down
into sections and stages, built in whole or in part, to produce
varying amounts of U-235 of varying degrees of enrichment.
The electromagnetic plant was even more flexible, since each
separator unit was practically independent of the other units.
The separation process consisted of loading a charge into a unit,
Electromagnetic Separation 201
running the unit for a while, then stopping it and removing the
product. To be sure, the units were built in groups, but most of
the controls were separate for each unit. This feature made it
possible to build the plant in steps and to start operating the first
part even before the second was begun. It was also possible to
change the design of subsequent units as construction proceeded;
within limits it was possible even to replace obsolescent units in
the early groups with new improved units.
NATURE AND ORGANIZATION OF DEVELOPMENT WORK
11.33. Construction of the first series of electromagnetic units
at Clinton began in March of 1943 and this part of the plant was
ready for operation in November 1943. The group at Berkeley
continued to improve the ion sources, the receivers, and the
auxiliary equipment, aiming always at greater ion currents. In
fact, Berkeley reports describe no less than seventy-one different
types of source and one hundred and fifteen different types of
receiver, all of which reached the design stage and most of which
were constructed and tested. As soon as the value of a given design
change was proved, every effort was made to incorporate it in
the designs of new units.
11.34. Such developments as these required constant inter-
change of information among laboratory, engineering, con-
struction, and operating groups. Fortunately the liaison was
excellent. The companies stationed representatives at Berkeley,
and members of the research group at Berkeley paid frequent
and prolonged visits to the plant at Clinton. In fact, some of the
research men were transferred to the payroll of the Tennessee
Eastman Company operating the plant at Clinton, and a group
of over one hundred physicists and research engineers still kept
on the Berkeley payroll were assigned to Clinton. Particularly
in the early stages of operation the Berkeley men stationed at
Clinton were invaluable as "trouble shooters" and hi instructing
operators. A section of the plant continued to be maintained as
a pilot unit for testing modified equipment and revised operating
procedures, and was run jointly by the Berkeley group and by
202 Electromagnetic Separation
Tennessee Eastman. In addition to the British group under
Oliphant already mentioned, there was a British group of chemists
at Clinton under J. W. Baxter.
CHEMICAL PROBLEMS
11.35. Originally, the uranium salts used as sources of vapor
for the ion-producing arcs had not been investigated with any
very great thoroughness at Berkeley, but as the process developed,
a good deal of work was done on these salts, and a search was
made for a uranium compound that would be better than that
originally used. Some valuable studies were also made on methods
of producing the compound chosen.
11.36. By far the most important chemical problem was the
recovery of the processed uranium compounds from the separa-
tion units. This recovery problem had two phases. In units of
the first stage it was essential to recover the separated uranium
from the receivers with maximum efficiency; whereas recovery
of the scattered unseparated uranium from other parts of the unit
was less important. But if higher stage units are used even the
starting material contains a high concentration of U-235, and it
is essential to recover all the material in the unit at the end of
each run, i.e., material remaining in the ion source and material
deposited on the accelerating electrodes, on the walls of the
magnet chamber, and on the receiver walls.
THE THERMAL DIFFUSION PLANT
11.37. For nearly a year the electromagnetic plant was the
only one in operation. Therefore the urge to increase its produc-
tion rate was tremendous. It was realized that any method of
enriching even slightly enriching the material to be fed into
the plant would increase the production rate appreciably. For
example, an electromagnetic unit that could produce a gram a
day of 40 per cent pure U-235 from natural uranium could
produce two grams a day of 80 per cent U-235 if the concentra-
tion of U-235 in the feed material was twice the natural concen-
tration (1.4 per cent instead of 0.7 per cent).
Electromagnetic Separation 203
11.38. We have already referred to the work done by P. H.
Abelson of the Naval Research Laboratory on the separation of
the uranium isotopes by thermal diffusion in a liquid compound
of uranium. By the spring of 1943 Abelson had set up a pilot
plant that accomplished appreciable separation of a considerable
quantity of uranium compound. It was therefore proposed that
a large-scale thermal diffusion plant should be constructed.
Such a plant would be cheaper than any of the other large-scale
plants, and it could be built more quickly. Its principal drawback
\\ as its enormous consumption of steam, which made it appear
impracticable for the whole job of separation.
11.39. Not only was a pilot plant already in operation at the
Naval Research Laboratory, but a second, somewhat larger
plant was under construction at the Philadelphia Navy Yard.
Through the cooperation of the Navy both the services of Abelson
and the plans for a large-scale plant were made available to the
Manhattan District. It was decided to erect the large-scale
thermal diffusion plant at Clinton (using steam from the power
plant constructed for the gaseous diffusion plant) and to use the
thermal-diffusion-plant product as feed material for the electro-
magnetic plant.
11.40. This new thermal diffusion plant was erected in amaz-
ingly short time during the late summer of 1944. In spite of some
disappointments, operation of this plant has succeeded in its
purpose of considerably increasing the production rate of the
electromagnetic plant. It has also stimulated work on the uranium
recovery problem. The future of this plant is uncertain. Oper-
ation of the gaseous-diffusion plant makes it difficult to get
enough steam to operate the thermal diffusion plant, but also
furnishes another user for its product.
MISCELLANEOUS PROBLEMS
11.41. Although the scientific and technical problems which
confronted the Berkeley groups were probably not as varied or
numerous as the problems encountered at Chicago and Columbia,
they were nevertheless numerous. Thus many problems arose in
204 Electromagnetic Separation
the designing of the electric power and control circuits, magnetic
fields, insulators, vacuum pumps, tanks, collectors, and sources.
Many equipment items had to be designed from scratch and then
mass-produced under high priority.
PRESENT STATUS
11.42. The electromagnetic separation plant was in large-scale
operation during the winter of 1944-1945, and produced U-235
of sufficient purity for use in atomic bombs. Its operating efficiency
is being continually improved. Research work is continuing
although on a reduced scale.
SUMMARY
11.43. In the early days of the uranium project, electro-
magnetic methods of isotope separation were rejected primarily
because of the expected effects of space-charge. In the fall of
1941 the question was reopened; experiments at Berkeley showed
that space-charge effects could be largely overcome. Conse-
quently a large-scale program for the development of electro-
magnetic methods was undertaken.
11.44. Of the various types of electromagnetic methods pro-
posed, the calutron (developed at Berkeley) received principal
attention. Two other novel methods were studied, one at Berkeley
and one at Princeton. The calutron mass separator consists of an
ion source from which a beam of uranium ions is drawn by an
electric field, an accelerating system in which the ions are
accelerated to high velocities, a magnetic field in which the ions
travel in semicircles of radius depending on ion mass, and a
receiving system. The principal problems of this method involved
the ion source, accelerating system, divergence of the ion beam,
space charge, and utilization of the magnetic field. The chief
advantages of the calutron were large separation factor, small
hold-up, short start-up time, and flexibility of operation. By the
fall of 1942 sufficient progress had been made to justify authoriza-
tion of plant construction, and a year later the first plant units
were ready for trial at the Clinton Engineer Works in Tennessee.
Electromagnetic Separation 205
11.45. Research and development work on the calutron were
carried out principally at the Radiation Laboratory of the Uni-
versity of California, under the direction of Lawrence. Westing-
house, General Electric, and Allis Chalmers constructed a
majority of the parts; Stone and Webster built the plant, and
Tennessee Eastman operated it.
11.46. Since the calutron separation method was one of batch
operations in a large number of largely independent units, it was
possible to introduce important improvements even after plant
operation had begun.
11.47. In the summer of 1944 a thermal-diffusion separation
plant was built at the Clinton Engineer Works to furnish en-
riched feed material for the electromagnetic plant and thereby
increase the production rate of this latter plant. The design of the
thermal-diffusion plant was based on the results of research car-
ried out at the Naval Research Laboratory and on the pilot plant
built by the Navy Department at the Philadelphia Navy Yard.
11.48. Although research work on the calutron was started
later than on the centrifuge and diffusion systems, the calutron
plant was the first to produce large amounts of the separated
isotopes of uranium.
CHAPTER XII. THE WORK ON THE
ATOMIC BOMB
THE OBJECTIVE
12.1. The entire purpose of the work described in the preceding
chapters was to explore the possibility of creating atomic bombs
and to produce the concentrated fissionable materials which
would be required in such bombs. In the present chapter, the last
stage of the work will be described the development at Los
Alamos of the atomic bomb itself. As in other parts of the project,
there are two phases to be considered: the organization, and the
scientific and technical work itself. The organization will be
described briefly; the remainder of the chapter will be devoted
to the scientific and technical problems. Security considerations
prevent a discussion of many of the most important phases of this
work.
HISTORY AND ORGANIZATION
12.2. The project reorganization that occurred at the begin-
ning of 1942, and the subsequent gradual transfer of the work
from OSRD auspices to the Manhattan District have been
described in Chapter V. It will be recalled that the responsibili-
ties of the Metallurgical Laboratory at Chicago originally
included a preliminary study of the physics of the atomic bomb.
Some such studies were made in 1941; and early in 1942
G. Breit got various laboratories (see Chapter VI, paragraph
6.38) started on the experimental study of problems that had to
be solved before progress could be made on bomb design. As has
been mentioned in Chapter VI, J. R. Oppenheimer of the Uni-
versity of California gathered a group together in the summer of
1942 for further theoretical investigation and also undertook to
coordinate this experimental work. This group was officially
206
Work on the Atomic Bomb 207
under the Metallurgical Laboratory but the theoretical group
did most of its work at the University of California. By the end
of the summer of 1 942, when General L. R. Groves took charge
of the entire project, it was decided to expand the work consider-
ably, and, at the earliest possible time, to set up a separate
laboratory.
12.3. In the choice of a site for this atomic-bomb laboratory,
the all-important considerations were secrecy and safety. It was
therefore decided to establish the laboratory in an isolated loca-
tion and to sever unnecessary connection with the outside world.
12.4. By November 1942 a site had been chosen at Los
Alamos, New Mexico. It was located on a mesa about 30 miles
from Santa Fe. One asset of this site was the availability of con-
siderable area for proving grounds, but initially the only struc-
tures on the site consisted of a handful of buildings which once
constituted a small boarding school. There was no laboratory,
no library, no shop, no adequate power plant. The sole means of
approach was a winding mountain road. That the handicaps of
the site were overcome to a considerable degree is a tribute to
the unstinting efforts of the scientific and military personnel.
12.5. J. R. Oppenheimer has been director of the laboratory
from the start. He arrived at the site in March 1943, and was
soon joined by groups and individuals from Princeton Univer-
sity, University of Chicago, University of California, University
of Wisconsin, University of Minnesota, and elsewhere. With the
vigorous support of General L. R. Groves, J. B. Conant, and
others, Oppenheimer continued to gather around him scientists
of recognized ability, so that the end of 1 944 found an extraordi-
nary galaxy of scientific stars gathered on this New Mexican
mesa. The recruiting of junior scientific personnel and technicians
was more difficult, since for such persons the disadvantages of the
site were not always counterbalanced by an appreciation of the
magnitude of the goal; the use of Special Engineer Detachment
personnel improved the situation considerably.
12.6. Naturally, the task of assembling the necessary apparatus,
machines, and equipment was an enormous one. Three carloads
208 Work on the Atomic Bomb
of apparatus from the Princeton project filled some of the most
urgent requirements. A cyclotron from Harvard, two Van de
Graaff generators from Wisconsin, and a Cockcroft-Walton
high- voltage device from Illinois soon arrived. As an illustration
of the speed with which the laboratory was set up, we may record
that the bottom pole piece of the cyclotron magnet was not laid
until April 14, 1943, yet the first experiment was performed in
early July. Other apparatus was acquired in quantity; subsidiary
laboratories were built. Today this is probably the best-equipped
physics research laboratory in the world.
12.7. The laboratory was financed under a contract between
the Manhattan District and the University of California.
STATE OF KNOWLEDGE IN APRIL 1943
GENERAL DISCUSSION OF THE PROBLEM
12.8. In Chapter II we stated the general conditions required
to produce a self-sustaining chain reaction. It was pointed out
that there are four processes competing for neutrons: (1) the
capture of neutrons by uranium which results in fission; (2)
non-fission capture by uranium; (3) non-fission capture by
impurities; and (4) escape of neutrons from the system. Therefore
the condition for obtaining such a chain reaction is that process
(1) shall produce as many new neutrons as are consumed or lost
in all four of the processes. It was pointed out that (2) may be
reduced by removal of U-238 or by the use of a lattice and
moderator, that (3) may be reduced by achieving a high degree
of chemical purity, and that (4) may be reduced (relatively) by
increasing the size of the system. In our earlier discussions of
chain reactions it was always taken for granted that the chain-
reacting system must not blow up. Now we want to consider how
to make it blow up.
12.9. By definition, an explosion is a sudden and violent release
of a large amount of energy in a small region. To produce an
efficient explosion in an atomic bomb, the parts of the bomb
must not become appreciably separated before a substantial
Work on the Atomic Bomb 209
fraction of the available nuclear energy has been released, since
expansion leads to increased escape of neutrons from the system
and thus to premature termination of the chain reaction. Stated
differently, the efficiency of the atomic bomb will depend on the
ratio of (a) the speed with which neutrons generated by the first
fissions get into other nuclei and produce further fission, and (b)
the speed with which the bomb flies apart. Using known prin-
ciples of energy generation, temperature and pressure rise, and
expansion of solids and vapors, it was possible to estimate the
order of magnitude of the time interval between the beginning
and end of the nuclear chain reaction. Almost all the technical
difficulties of the project come from the extraordinary brevity
of this time interval.
12.10. In earlier chapters we stated that no self-sustaining
chain reaction could be produced in a block of pure uranium
metal, no matter how large, because of parasitic capture of the
neutrons by U-238. This conclusion has been borne out by various
theoretical calculations and also by direct experiment. For pur-
poses of producing a non-explosive pile, the trick of using a lattice
and a moderator suffices by reducing parasitic capture suffi-
ciently. For purposes of producing an explosive unit, however,
it turns out that this process is unsatisfactory on two counts. First,
the thermal neutrons take so long (so many micro-seconds) to
act that only a feeble explosion would result. Second, a pile is
ordinarily far too big to be transported. It is therefore necessary
to cut down parasitic capture by removing the greater part of the
U-238 or to use plutonium.
12.11. Naturally, these general principles and others had
been well established before the Los Alamos project was set up.
CRITICAL SIZE
12.12. The calculation of the critical size of a chain-reacting
unit is a problem that has already been discussed in connection
with piles. Although the calculation is simpler for a homogeneous
metal unit than for a lattice, inaccuracies remained in the course
of the early work, both because of lack of accurate knowledge of
210 Work on the Atomic Bomb
constants and because of mathematical difficulties. For example,
the scattering, fission, and absorption cross sections of the nuclei
involved all vary with neutron velocity. The details of such varia-
tion were not known experimentally and were difficult to take
into account in making calculations. By the spring of 1 943 several
estimates of critical size had been made using various methods of
calculation and using the best available nuclear constants, but
the limits of error remained large.
THE REFLECTOR OR TAMPER
12.13. In a uranium-graphite chain-reacting pile the critical
size may be considerably reduced by surrounding the pile with
a layer of graphite, since such an envelope "reflects" many neu-
trons back into the pile. A similar envelope can be used to reduce
the critical size of the bomb, but here the envelope has an addi-
tional role: its very inertia delays the expansion of the reacting
material. For this reason such an envelope is often called a
tamper. Use of a tamper clearly makes for a longer lasting, more
energetic, and more efficient explosion. The most effective tamper
is the one having the highest density; high tensile strength turns
out to be unimportant. It is a fortunate coincidence that mate-
rials of high density are also excellent as reflectors of neutrons.
EFFICIENCY
12.14. As has already been remarked, the bomb tends to fly
to bits as the reaction proceeds and this tends to stop the reaction.
To calculate how much the bomb has to expand before the
reaction stops is relatively simple. The calculation of how long
this expansion takes and how far the reaction goes in that time
is much more difficult.
12.15. While the effect of a tamper is to increase the efficiency
both by reflecting neutrons and by delaying the expansion of
the bomb, the effect on the efficiency is not as great as on the
critical mass. The reason for this is that the process of reflection
is relatively time-consuming and may not occur extensively
before the chain reaction is terminated.
Work on the Atomic Bomb 21 1
DETONATION AND ASSEMBLY
12.16. As stated in Chapter II, it is impossible to prevent a
chain reaction from occurring when the size exceeds the critical
size. For there are always enough neutrons (from cosmic rays,
from spontaneous fission reactions, or from alpha-particle-in-
duced reactions in impurities) to initiate the chain. Thus until
detonation is desired, the bomb must consist of a number of
separate pieces each one of which is below the critical size either
by reason of small size or unfavorable shape. To produce
detonation, the parts of the bomb must be brought together
rapidly. In the course of this assembly process the chain reaction
is likely to start because of the presence of stray neutrons
before the bomb has reached its most compact (most reactive)
form. Thereupon the explosion tends to prevent the bomb from
reaching that most compact form. Thus it may turn out that
the explosion is so inefficient as to be relatively useless. The
problem, therefore, is two-fold: (1) to reduce the time of assembly
to a minimum; and (2) to reduce the number of stray (pre-
detonation) neutrons to a minimum.
12.17. Some consideration was given to the danger of pro-
ducing a "dud" or a detonation so inefficient that even the bomb
itself would not be completely destroyed. This would, of course,
present the enemy with a supply of highly valuable material.
EFFECTIVENESS
12.18. In Chapters II and IV it was pointed out that the
amount of energy released was not the sole criterion of the value
of a bomb. There was no assurance that one uranium bomb
releasing energy equal to the energy released by 20,000 tons of
TNT would be as effective in producing military destruction as,
say, 10,000 two-ton bombs. In fact, there were good reasons to
believe that the destructive effect per calorie released decreases
as the total amount of energy released increases. On the other
hand, in atomic bombs the total amount of energy released per
kilogram of fissionable material (i.e., the efficiency of energy
212 Work on the Atomic Bomb
release) increases with the size of the bomb. Thus the optimum
size of the atomic bomb was not easily determined. A tactical
aspect that complicates the matter further is the advantage of
simultaneous destruction of a large area of enemy territory. In
a complete appraisal of the effectiveness of an atomic bomb,
attention must also be given to effects on morale.* The bomb
is detonated in combat at such a height above the ground as to
give the maximum blast effect against structures, and to dis-
seminate the radioactive products as a cloud. On account of the
height of the explosion practically all the radioactive products
are carried upward in the ascending column of hot air and dis-
persed harmlessly over a wide area. Even in the New Mexico
test, where the height of explosion was necessarily low, only a
very small fraction of the radioactivity was deposited immediately
below the bomb.
METHOD OF ASSEMBLY
12.19. Since estimates had been made of the speed that would
bring together subcritical masses of U-235 rapidly enough to
avoid predetonation, a good deal of thought had been given to
practical methods of doing this. The obvious method of very
rapidly assembling an atomic bomb was to shoot one part as a
projectile in a gun against a second part as a target. The projectile
mass, projectile speed, and gun caliber required were not far
from the range of standard ordnance practice, but novel problems
were introduced by the importance of achieving sudden and
perfect contact between projectile and target, by the use of
tampers, and by the requirement of portability. None of these
technical problems had been studied to any appreciable extent
prior to the establishment of the Los Alamos laboratory.
12.20. It had also been realized that schemes probably might
be devised whereby neutron absorbers could be incorporated in
the bomb in such a way that they would be rendered less effective
by the initial stages of the chain reactions. Thus the tendency for
* The rest of this paragraph is from a War Department release subsequent
to the first use of atomic bombs against Japan.
Work on the Atomic Bomb 213
the bomb to detonate prematurely and inefficiently would be
minimized. Such devices for increasing the efficiency of the bomb
are called auto-catalytic.
SUMMARY OF KNOWLEDGE AS OF APRIL 1943
12.21. In April 1943 the available information of interest in
connection with the design of atomic bombs was preliminary
and inaccurate. Further and extensive theoretical work on critical
size, efficiency, effect of tamper, method of detonation, and
effectiveness was urgently needed. Measurements of the nuclear
constants of U-235, plutonium, and tamper material had to be
extended and improved. In the cases of U-235 and plutonium,
tentative measurements had to be made using only minute
quantities until larger quantities became available.
12.22. Besides these problems in theoretical and experimental
physics, there was a host of chemical, metallurgical, and technical
problems that had hardly been touched. Examples were the
purification and fabrication of U-235 and plutonium, and the
fabrication of the tamper. Finally, there were problems of instan-
taneous assembly of the bomb that were staggering in their
complexity.
THE WORK OF THE LABORATORY
INTRODUCTION
12.23. For administrative purposes the scientific staff at Los
Alamos was arranged in seven divisions, which have been re-
arranged at various times. During the spring of 1 945 the divisions
were: Theoretical Physics Division under H. Bethe, Experi-
mental Nuclear Physics Division under R. R. Wilson, Chemistry
and Metallurgy Division under J. W. Kennedy and C. S. Smith,
Ordnance Division under Capt. W. S. Parsons (USN), Explosives
Division under G. B. Kistiakowsky, Bomb Physics Division under
R. F. Bacher, and an Advanced Development Division under
E. Fermi. All the divisions reported to J. R. Oppenheimer,
Director of the Los Alamos Laboratory who has been assisted in
214 Work on the Atomic Bomb
coordinating the research by S. K. Allison since December 1944.
J. Chad wick of England and N. Bohr of Denmark spent a great
deal of time at Los Alamos and gave invaluable advice. Chadwick
was the head of a British delegation which contributed materially
to the success of the laboratory. For security reasons, most of the
work of the laboratory can be described only in part.
THEORETICAL PHYSICS DIVISION
12.24. There were two considerations that gave unusual
importance to the work of the Theoretical Physics Division under
H. Bethe. The first of these was the necessity for effecting simul-
taneous development of everything from the fundamental mate-
rials to the method of putting them to use all despite the virtual
unavailability of the principal materials (U-235 and plutonium)
and the complete novelty of the processes. The second considera-
tion was the impossibility of producing (as for experimental
purposes) a "small-scale" atomic explosion by making use of
only a small amount of fissionable material. (No explosion occurs
at all unless the mass of the fissionable material exceeds the
critical mass.) Thus it was necessary to proceed from data ob-
tained in experiments on infinitesimal quantities of materials
and to combine it with the available theories as accurately as
possible in order to make estimates as to what would happen in
the bomb. Only in this way was it possible to make sensible plans
for the other parts of the project, and to make decisions on design
and construction without waiting for elaborate experiments on
large quantities of material. To take a few examples, theoretical
work was required in making rough determinations of the dimen-
sions of the gun, in guiding the metallurgists in the choice of
tamper materials, and in determining the influence of the purity
of the fissionable material on the efficiency of the bomb.
12.25. The determination of the critical size of the bomb was
one of the main problems of the Theoretical Physics Division. In
the course of time, several improvements were made in the
theoretical approach whereby it was possible to take account of
practically all the complex phenomena involved. It was at first
Work on the Atomic Bomb 215
considered that the diffusion of neutrons was similar to the diffu-
sion of heat, but this naive analogy had to be forsaken. In the
early theoretical work the assumptions were made that the
neutrons all had the same velocity and all were scattered iso-
tropically. A method was thus developed which permitted cal-
culation of the critical size for various shapes of the fissionable
material provided that the mean free path of the neutrons was
the same in the tamper material as in the fissionable material.
This method was later improved first by taking account of the
angular dependence of the scattering and secondly by allowing
for difference in mean free path in core and tamper materials.
Still later, means were found of taking into account the effects
of the distribution in velocity of the neutrons, the variations of
cross sections with velocity, and inelastic scattering in the core
and tamper materials. Thus it became possible to compute
critical sizes assuming almost any kind of tamper material.
12.26. The rate at which the neutron density decreases in
bomb models which are smaller than the critical size can be cal-
culated, and all the variables mentioned above can be taken into
account. The rate of approach to the critical condition as the
projectile part of the bomb moves toward the target part of the
bomb has been studied by theoretical methods. Furthermore,
the best distribution of fissionable material in projectile and target
was determined by theoretical studies.
12.27. Techniques were developed for dealing with set-ups in
which the number of neutrons is so small that a careful statistical
analysis must be made of the effects of the neutrons. The most
important problem in this connection was the determination of
the probability that, when a bomb is larger than critical size,
a stray neutron will start a continuing chain reaction. A related
problem was the determination of the magnitude of the fluctua-
tions in neutron density in a bomb whose size is close to the critical
size. By the summer of 1945 many such calculations had been
checked by experiments.
12.28. A great deal of theoretical work was done on the equa-
tion of state of matter at the high temperatures and pressures to
216 Work on the Atomic Bomb
be expected in the exploding atomic bombs. The expansion of
the various constituent parts of the bomb during and after the
moment of chain reaction has been calculated. The effects of
radiation have been investigated in considerable detail.
12.29. Having calculated the energy that is released in the
explosion of an atomic bomb, one naturally wants to estimate
the military damage that will be produced. This involves analysis
of the shock waves in air and in earth, the determination of the
effectiveness of a detonation beneath the surface of the ocean, etc.
12.30. In addition to all the work mentioned above, a con-
siderable amount of work was done in evaluating preliminary
experiments. Thus an analysis was made of the back-scattering
of neutrons by the various tamper materials proposed. An
analysis was also made of the results of experiments on the
multiplication of neutrons in subcritical amounts of fissionable
material.
EXPERIMENTAL NUCLEAR PHYSICS DIVISION
12.31. The experiments performed by the Experimental
Nuclear Physics group at Los Alamos were of two kinds: "differ-
ential" experiments as for determining the cross section for fission
of a specific isotope by neutrons of a specific velocity, and "in-
tegral" experiments as for determining the average scattering of
fission neutrons from an actual tamper.
12.32. Many nuclear constants had already been determined
at the University of Chicago Metallurgical Laboratory and else-
where, but a number of important constants were still undeter-
mined especially those involving high neutron velocities. Some
of the outstanding questions were the following:
1. What are the fission cross sections of U-234, U-235, U-238,
Pu-239, etc.? How do they vary with neutron velocity?
2. What are the elastic scattering cross sections for the same
nuclei (also for nuclei of tamper materials)? How do they vary
with neutron velocity?
3. What are the inelastic cross sections for the nuclei referred
to above?
Work on the Atomic Bomb 217
4. Wh'at are the absorption cross sections for processes other
than fission?
5. How many neutrons are emitted per fission in the case of
each of the nuclei referred to above?
6. What is the full explanation of the fact that the number of
neutrons emitted per fission is not a whole number?
7. What is the initial energy of the neutrons produced by
fission?
8. Does the number or energy of such neutrons vary with the
speed of the incident neutrons?
9. Are fission neutrons emitted immediately?
10. WTiat is the probability of spontaneous fission of the various
fissionable nuclei?
12.33. In addition to attempting to find the answers to these
questions the Los Alamos Experimental Nuclear Physics Division
investigated many problems of great scientific interest which
were expected to play a role in their final device. Whether or not
this turned out to be the case, the store of knowledge thus accumu-
lated by the Division forms an integral and invaluable part of all
thinking on nuclear problems.
12.34. Experimental Methods. The earlier chapters contain little
or no discussion of experimental techniques except those for the
observing of fast (charged) particles (See Appendix 1.). To obtain
answers to the ten questions posed above, we should like to be
able to:
(1) determine the number of neutrons of any given energy;
(2) produce neutrons of any desired energy;
(3) determine the angles of deflection of scattered neutrons;
(4) determine the number of fissions occurring;
(5) detect other consequences of neutron absorption, e.g.,
artificial radioactivity.
We shall indicate briefly how such observations are made.
12.35. Detection of Neutrons. There are three ways in which
neutrons can be detected: by the ionization produced by light
atomic nuclei driven forward at high speeds by elastic collisions
218 Work on the Atomic Bomb
with neutrons, by the radioactive disintegration of unstable
nuclei formed by the absorption of neutrons, and by fission
resulting from neutron absorption. All three processes lead to
the production of ions and the resulting ionization may be
detected using electroscopes, ionization chambers, Geiger-Miiller
counters, Wilson cloud chambers, tracks in photographic emul-
sion, etc.
12.36. While the mere detection of neutrons is not difficult,
the measurement of the neutron velocities is decidedly more so.
The Wilson cloud chamber method and the photographic
emulsion method give the most direct results but are tedious to
apply. More often various combinations of selective absorbers
are used. Thus, for example, if a foil known to absorb neutrons
of only one particular range of energies is inserted in the path of
the neutrons and is then removed, its degree of radioactivity is
presumably proportional to the number of neutrons in the
particular energy range concerned. Another scheme is to study
the induced radioactivity known to be produced only by neutrons
whose energy lies above a certain threshold.
12.37. One elegant scheme for studying the effects of neutrons
of a single, arbitrarily-selected velocity is the "time of flight"
method. In this method a neutron source is modulated, i.e., the
source is made to emit neutrons in short "bursts" or "pulses."
In each pulse there are a great many neutrons of a very wide
range of velocities. The target material and the detector are
situated a considerable distance from the source (several feet
or yards from it). The detector is "modulated" also, and with
the same periodicity. The timing or phasing is made such that
the detector is responsive only for a short interval beginning a
certain time after the pulse of neutrons leaves the source. Thus
any effects recorded by the detector (e.g., fissions in a layer of
uranium deposited on an inner surface of an ionization chamber)
are the result only of neutrons that arrive just at the moment of
responsivity and therefore have travelled from the source in a
certain time interval. In other words, the measured effects are
due only to the neutrons having the appropriate velocity.
Work on the Atomic Bomb 219
12.38. Production of Neutrons. All neutrons are produced as the
result of nuclear reactions, and their initial speed depends on the
energy balance of the particular reaction. If the reaction is
endothermic, that is, if the total mass of the resultant particles is
greater than that of the initial particles, the reaction does not
occur unless the bombarding particle has more than the "thresh-
old"' kinetic energy. At higher bombarding energies the kinetic
energy- of the resulting particles, specifically of the neutrons, goes
up with the increase of kinetic energy of the bombarding particle
above the threshold value. Thus the Li 7 (p, n)Be 7 reaction absorbs
1.6 Mev energy since the product particles are heavier than the
initial particles. Any further energy of the incident protons goes
into kinetic energy of the products so that the maximum speed
of the neutrons produced goes up with the speed of the incident
protons. However, to get neutrons of a narrow range of speed, a
thin target must be used, the neutrons must all come off at the
same angle, and the protons must all strike the target with the
same speed.
12.39. Although the same energy and momentum conserva-
tion laws apply to exothermic nuclear reactions, the energy
release is usually large compared to the kinetic energy of the
bombarding particles and therefore essentially determines the
neutron speed. Often there are several ranges of speed from
the same reaction. There are some reactions that produce very
high energy neutrons (nearly 15 Mev).
12.40. Since there is a limited number of nuclear reactions
usable for neutron sources, there are only certain ranges of
neutron speeds that can be produced originally. There is no
difficulty about slowing down neutrons, but it is impossible to
slow them down uniformly, that is, without spreading out the
velocity distribution. The most effective slowing-down scheme is
the use of a moderator, as in the graphite pile; in fact, the pile
itself is an excellent source of thermal (i.e., very low speed) or
nearly thermal neutrons.
12.41. Determination of Angles of Deflection. The difficulties in
measuring the angles of deflection of neutrons are largely of
220 Work on the Atomic Bomb
intensity and interpretation. The number of neutrons scattered
in a particular direction may be relatively small, and the "scat-
tered" neutrons nearly always include many strays not coming
from the intended target.
12.42. Determination of Number of Fissions. The determination
of the number of fissions which are produced by neutrons or
occur spontaneously is relatively simple. lonization chambers,
counter tubes, and many other types of detectors can be used.
12.43. Detection of Products of Capture of Neutrons. Often it is
desirable to find in detail what has happened to neutrons that
are absorbed but have not produced fission, e.g., resonance or
"radiative" capture of neutrons by U-238 to form U-239 which
leads to the production of plutonium. Such studies usually
involve a combination of microchemical separations and radio-
activity analyses.
12.44. Some Experiments on Nuclear Constants. By the time that
the Los Alamos laboratory had been established, a large amount
of work had been done on the effects of slow neutrons on the
materials then available. For example, the thermal-neutron
fission cross section of natural uranium had been evaluated, and
similarly for the separated isotopes of uranium and for plutonium.
Some data on high-speed-neutron fission cross sections had been
published, and additional information was available in project
laboratories. To extend and improve such data, Los Alamos per-
fected the use of the Van de Graaff generator for the Li 7 (p, n)Be 7
reaction, so as to produce neutrons of any desired energy lying
in the range from 3,000 electron volts to two million electron
volts. Success was also achieved in modulating the cyclotron
beam and developing the neutron time-of-flight method to
produce effects of many speed intervals at once. Special methods
were devised for filling in the gaps in neutron energy range.
Particularly important was the refinement of measurement made
possible as greater quantities of U-235, U-238 and plutonium
began to be received. On the whole, the value of the cross section
for fission as a function of neutron energy from practically zero
electron volts to three million electron volts is now fairly well
known for these materials.
Work on the Atomic Bomb 221
12.45. Some Integral Experiments. Two "integral experiments"
(experiments on assembled or integrated systems comprising
fissionable material, reflector, and perhaps moderator also) may
be described. In the first of these integral experiments a chain-
reacting system was constructed which included a relatively
large amount of U-235 in liquid solution. It was designed to
operate at a very low power level, and it had no cooling system.
Its purpose was to provide verification of the effects predicted
for reacting systems containing enriched U-235. The results were
very nearly as expected.
12.46. The second integral experiment was carried out on a
pile containing a mixture of uranium and a hydrogenous moder-
ator. In this first form, the pile was thus a slow-neutron chain-
reacting pile. The pile was then rebuilt using less hydrogen. In
this version of the pile, fast-neutron fission became important.
The pile was rebuilt several more times, less hydrogen being
used each time. By such a series of reconstructions, the reaction
character was successively altered, so that thermal neutron fission
became less and less important while fast neutron fission became
more and more important approaching the conditions to be
found in the bomb.
12.47. Summary of Results on Nuclear Physics. The nuclear con-
stants of U-235, U-238, and plutonium have been measured with
a reasonable degree of accuracy over the range of neutron energies
from thermal to three million electron volts. In other words,
questions 1, 2, 3, 4, and 5 of the ten questions posed at the
beginning of this section have been answered. The fission spectrum
(question 7) for U-235 and Pu-239 is reasonably well known.
Spontaneous fission (question 10) has been studied for several
types of nuclei. Preliminary results on questions 6, 8, and 9,
involving details of the fission process, have been obtained.
CHEMISTRY AND METALLURGY DIVISION
12.48. The Chemistry and Metallurgy Division of the Los
Alamos Laboratory was under the joint direction of J. W.
Kennedy and C. S. Smith. It was responsible for final purification
of the enriched fissionable materials, for fabrication of the bomb
222 Work on the Atomic Bomb
core, tamper, etc., and for various other matters. In all this
division's work on enriched fissionable materials especial care
had to be taken not to lose any appreciable amounts of the ma-
terials which are worth much more than gold. Thus the pro-
cedures already well-established at Chicago and elsewhere for
purifying and fabricating natural uranium were often not satis-
factory for handling highly-enriched samples of U-235.
ORDNANCE, EXPLOSIVES, AND BOMB PHYSICS DIVISIONS
12.49. The above account of the work of the Theoretical
Physics, Experimental Nuclear Physics, and Chemistry and
Metallurgy Divisions is very incomplete because important
aspects of this work cannot be discussed for reasons of security.
For the same reasons none of the work of the Ordnance, Explo-
sives, and Bomb Physics Divisions can be discussed at all.
SUMMARY
12.50. In the spring of 1943 an entirely new laboratory was
established at Los Alamos, New Mexico, under J. R. Oppen-
heimer for the purpose of investigating the design and construc-
tion of the atomic bomb, from the stage of receipt of U-235 or
plutonium to the stage of use of the bomb. The new laboratory
improved the theoretical treatment of design and performance
problems, refined and extended the measurements of the nuclear
constants involved, developed methods of purifying the materials
to be used, and, finally, designed and constructed operable atomic
bombs.
CHAPTER XIII. GENERAL SUMMARY
PRESENT OVERALL STATUS
13.1. As the result of the labors of the Manhattan District
organization in Washington and in Tennessee, of the scientific
groups at Berkeley, Chicago, Columbia, Los Alamos, and else-
where, of the industrial groups at Clinton, Hanford, and many
other places, the end of June 1945 finds us expecting from day to
day to hear of the explosion of the first atomic bomb devised by
man. All the problems are believed to have been solved at least
well enough to make a bomb practicable. A sustained neutron
chain reaction resulting from nuclear fission has been demon-
strated; the conditions necessary to cause such a reaction to
occur explosively have been established and can be achieved;
production plants of several different types are in operation,
building up a stock pile of the explosive material. Although we
do not know when the first explosion will occur nor how effective
it will be, announcement of its occurrence will precede the pub-
lication of this report. Even if the first attempt is relatively
ineffective, there is little doubt that later efforts will be highly
effective; the devastation from a single bomb is expected to be
comparable to that of a major air raid by usual methods.
13.2. A weapon has been developed that is potentially destruc-
tive beyond the wildest nightmares of the imagination; a w r eapon
so ideally suited to sudden unannounced attack that a country's
major cities might be destroyed overnight by an ostensibly
friendly power. This weapon has been created not by the devilish
inspiration of some warped genius but by the arduous labor of
thousands of normal men and women working for the safety of
their country. Many of the principles that have been used were
well known to the international scientific world in 1940. To
develop the necessary industrial processes from these principles
223
224 General Summary
has been costly in time, effort, and money, but the processes
which we selected for serious effort have worked and several
that we have not chosen could probably be made to work. We
have an initial advantage in time because, so far as we know,
other countries have not been able to carry out parallel develop-
ments during the war period. We also have a general advantage
in scientific and particularly in industrial strength, but such an
advantage can easily be thrown away.
13.3. Before the surrender of Germany there was always a
chance that German scientists and engineers might be developing
atomic bombs which would be sufficiently effective to alter the
course of the war. There was therefore no choice but to work on
them in this country. Initially many scientists could and did hope
that some principle would emerge which would prove that
atomic bombs were inherently impossible. This hope has faded
gradually; fortunately in the same period the magnitude of the
necessary industrial effort has been demonstrated so that the fear
of German success weakened even before the end came. By the
same token, most of us are certain that the Japanese cannot
develop and use this weapon effectively.
PROGNOSTICATION
13.4. As to the future, one may guess that technical develop-
ments will take place along two lines. From the military point
of view it is reasonably certain that there will be improvements
both in the processes of producing fissionable material and in its
use. It is conceivable that totally different methods may be dis-
covered for converting matter into energy since it is to be remem-
bered that the energy released in uranium fission corresponds to
the utilization of only about one-tenth of one per cent of its mass.
Should a scheme be devised for converting to energy even as
much as a few per cent of the matter of some common material,
civilization would have the means to commit suicide at will.
13.5. The possible uses of nuclear energy are not all destruc-
tive, and the second direction in which technical development can
be expected is along the paths of peace. In the fall of 1 944 General
General Summary 225
Groves appointed a committee to look into these possibilities as
well as those of military significance. This committee (Dr. R. C.
Tolman, chairman; Rear Admiral E. W. Mills (USN) with
Captain T. A. Solberg (USN) as deputy, Dr. W. K. Lewis, and
Dr. H. D. Smyth) received a multitude of suggestions from men
on the various projects, principally along the lines of the use of
nuclear energy for power and the use of radioactive by-products
for scientific, medical, and industrial purposes. While there was
general agreement that a great industry might eventually arise,
comparable, perhaps, with the electronics industry, there was
disagreement as to how rapidly such an industry would grow; the
consensus was that the growth would be slow over a period of
many years. At least there is no immediate prospect of running
cars with nuclear power or lighting houses with radioactive
lamps although there is a good probability that nuclear power
for special purposes could be developed within ten years and that
plentiful supplies of radioactive materials can have a profound
effect on scientific research and perhaps on the treatment of
certain diseases in a similar period.
PLANNING FOR THE FUTURE
13.6. During the war the effort has been to achieve the maxi-
mum military results. It has been apparent for some time that
some sort of government control and support in the field of
nuclear energy must continue after the war. Many of the men
associated with the project have recognized this fact and have
come forward with various proposals, some of which were con-
sidered by the Tolman Committee, although it was only a
temporary advisory committee reporting to General Groves. An
interim committee at a high level is now engaged in formulating
plans for a continuing organization. This committee is also
discussing matters of general policy about which many of the
more thoughtful men on the project have been deeply concerned
since the work was begun and especially since success became
more and more probable.
226 General Summary
THE QUESTIONS BEFORE THE PEOPLE
13.7. We find ourselves with an explosive which is far from
completely perfected. Yet the future possibilities of such explosives
are appalling, and their effects on future wars and international
affairs are of fundamental importance. Here is a new tool for
mankind, a tool of unimaginable destructive power. Its develop-
ment raises many questions that must be answered in the near
future.
13.8. Because of the restrictions of military security there has
been no chance for the Congress or the people to debate such
questions. They have been seriously considered by all concerned
and vigorously debated among the scientists, and the conclusions
reached have been passed along to the highest authorities. These
questions are not technical questions; they are political and social
questions, and the answers given to them may affect all mankind
for generations. In thinking about them the men on the project
have been thinking as citizens of the United States vitally inter-
ested in the welfare of the human race. It has been their duty
and that of the responsible high government officials who were
informed to look beyond the limits of the present war and its
weapons to the ultimate implications of these discoveries. This
was a heavy responsibility. In a free country like ours, such
questions should be debated by the people and decisions must
be made by the people through their representatives. This is one
reason for the release of this report. It is a semi-technical report
which it is hoped men of science in this country can use to help
their fellow citizens in reaching wise decisions. The people of
the country must be informed if they are to discharge their
responsibilities wisely.
APPENDIX 7. METHODS OF OBSERVING FAST
PARTICLES FROM NUCLEAR REACTIONS
In Chapter I we pointed out the importance of ionization in the
study of radioactivity and mentioned the electroscope. In this
appendix we shall mention one method of historical importance
comparable with the electroscope but no longer used, and then
we shall review the various methods now in use for observing
alpha particles, beta particles (or positrons), gamma rays, and
neutrons, or their effects.
SCINTILLATIONS
The closest approach that can be made to "seeing" an atom is
to see the bright flash of light that an alpha particle or high-speed
proton makes when it strikes a fluorescent screen. All that is
required is a piece of glass covered with zinc sulphide, a low-
power microscope, a dark room, a well-rested eye, and a source
of alpha particles. Most of Rutherford's famous experiments,
including that mentioned in paragraph 1.17, involved "counting"
scintillations but the method is tedious and, as far as the author
knows, has been entirely superseded by electrical methods.
THE PROCESS OF IONIZATION
When a high-speed charged particle like an alpha particle or a
high-speed electron passes through matter, it disrupts the mole-
cules that it strikes by reason of the electrical forces between the
charged particle and the electrons in the molecule. If the material
is gaseous, the resultant fragments or ions may move apart and,
if there is an electric field present, the electrons knocked out of
the molecules move in one direction and the residual positive
ions in another direction. A beta particle with a million electron
volts energy will produce some 18,000 ionized atoms before it is
227
228 Appendix 1
stopped completely since on the average it uses up about 60 volts
energy in each ionizing collision. Since each ionization process
gives both a positive and a negative ion, there is a total of 36,000
charges set free by one high-speed electron, but since each charge
is only 1.6 X 10~ 19 coulomb, the total is only about 6 X 10~ 15
coulomb and is still very minute. The best galvanometer can be
made to measure a charge of about 10~ 10 coulomb. It is posssible
to push the sensitivity of an electrometer to about 10~ 16 coulomb,
but the electrometer is a very inconvenient instrument to use.
An alpha particle produces amounts of ionization comparable
with the beta particle. It is stopped more rapidly, but it produces
more ions per unit of path. A gamma ray is much less efficient
as an ionizer since the process is quite different. It does occa-
sionally set free an electron from a molecule by Compton scatter-
ing or the photoelectric effect, and this secondary electron has
enough energy to produce ionization. A neutron, as we have
already mentioned in the text, produces ionization only indirectly
by giving high velocity to a nucleus by elastic collision, or by
disrupting a nucleus with resultant ionization by the fragments.
If we are to detect the ionizing effects of these particles, we
must evidently use the resultant effect of a great many particles
or have very sensitive means of measuring electric currents.
THE ELECTROSCOPE
Essentially the electroscope determines to what degree the air
immediately around it has become conducting as the result of
the ions produced in it.
The simplest form of electroscope is a strip of gold leaf a few
centimeters long, suspended by a hinge from a vertical insulated
rod. If the rod is charged, the gold leaf also takes up the same
charge and stands out at an angle as a result of the repulsion of
like charges. As the charge leaks away, the leaf gradually swings
down against the rod, and the rate at which it moves is a measure
of the conductivity of the air surrounding it.
A more rugged form of electroscope was devised by C. C.
Lauritsen, who substituted a quartz fiber for the gold leaf and
Observing Fast Particles 229
used the elasticity of the fiber as the restoring force instead of
gravity. The fiber is made conducting by a thin coating of metal.
Again the instrument is charged, and the fiber, after initial
deflection, gradually comes back to its uncharged position. The
position of the fiber is read in a low-power microscope. These
instruments can be made portable and rugged and fairly sensi-
tive. They are the standard field instrument for testing the level
of gamma radiation, particularly as a safeguard against dangerous
exposure.
IONIZATION CHAMBERS
An ionization chamber measures the total number of ions
produced directly in it. It usually consists of two plane electrodes
between which there is a strong enough electric field to draw all
the ions to the electrodes before they recombine but not strong
enough to produce secondary ions as in the instruments we shall
describe presently.
By careful design and the use of sensitive amplifiers an ioniza-
tion chamber can measure a number of ions as low as that pro-
duced by a single alpha particle, or it can be used much like an
electroscope to measure the total amount of ionizing radiation
present instantaneously, or it can be arranged to give the total
amount of ionization that has occurred over a period of time.
PROPORTIONAL COUNTERS
While ionization chambers can be made which will respond
to single alpha particles, it is far more convenient to use a self-
amplifying device, that is, to make the ions originally produced
make other ions in the same region so that the amplifier circuits
need not be so sensitive.
In a proportional counter one of the electrodes is a fine wire
along the axis of the second electrode, which is a hollow cylinder.
The effect of the wire is to give strong electric field strengths close
to it even for relatively small potential differences between it
and the other electrode. This strong field quickly accelerates the
230 Appendix 1
primary ions formed by the alpha or beta particle or photon, and
these accelerated primary ions (particularly the electrons) in
turn form secondary ions in the gas with which the counter is
filled so that the total pulse of current is much increased.
It is possible to design and operate such counters in such a way
that the total number of ions formed is proportional to the num-
ber of primary ions formed. Thus after amplification a current
pulse can be seen on an oscilloscope, the height of which will
indicate how effective an ionizer the initial particle was. It is
quite easy to distinguish in this way between alpha particles and
beta particles and photons, and the circuits can be arranged to
count only the pulses of greater than a chosen magnitude. Thus
a proportional counter can count alpha particles against a back-
ground of betas or can even count only the alpha particles having
more than a certain energy.
GEIGER-MULLER COUNTERS
If the voltage on a proportional counter is raised, there comes
a point when the primary ions from a single alpha particle, beta
particle, or photon will set off a discharge through the whole
counter, not merely multiply the number of primary ions in the
region where they are produced. This is a trigger action and the
current is independent of the number of ions produced ; further-
more, the current would continue indefinitely if no steps were
taken to quench it. Quenching can be achieved entirely by
arranging the external circuits so that the voltage drops as soon
as current passes or by using a mixture of gases in the counter
which "poison" the electrode surface as soon as the discharge
passes and temporarily prevent the further emission of electrons,
or by combining both methods.
The Geiger-Muller counter was developed before the propor-
tional counter and remains the most sensitive instrument for
detecting ionizing radiation, but all it does is "count" any ionizing
radiation that passes through it whether it be an alpha particle,
proton, electron, or photon.
Observing Fast Particles 231
THE ART OF COUNTER MEASUREMENTS
It is one thing to describe the principles of various ionization
chambers, counters, and the like; quite another to construct and
operate them successfully.
First of all, the walls of the counter chamber must allow the
particles to enter the counter. For gamma rays this is a minor
problem, but for relatively low-speed electrons or positrons or for
alpha particles the walls of the counter must be very thin or
there must be thin windows.
Then there are great variations in the details of the counter
itself, spacing and size of electrodes, nature of the gas filling the
chamber, its pressure, and so on.
Finally, the interpretation of the resultant data is a tricky
business. The absorption of the counter walls and of any external
absorbers must be taken into account; the geometry of the
counter with relation to the source must be estimated to translate
observed counts into actual number of nuclear events; last but
not always least, statistical fluctuations must be considered since
all nuclear reactions are governed by probability laws.
THE WILSON CLOUD CHAMBER
There is one method of observing nuclear particles that depends
directly on ionization but is not an electrical method. It uses the
fact that supersaturated vapor will condense more readily on
ions than on neutral molecules. If air saturated with water vapor
is cooled by expansion just after an alpha particle has passed
through it, tiny drops of water condense on the ions formed by
the alpha particle and will reflect a bright light strongly enough
to be seen or photographed so that the actual path of the alpha
particle is recorded.
This method developed by C. T. R. Wilson in Cambridge,
England, about 1912 has been enormously useful in studying the
behavior of individual particles, alphas, protons, electrons,
positrons, mesotrons, photons, and the fast atoms caused by
collisions with alphas, protons, or neutrons. Unlike the scintilla-
232 Appendix 1
tion method, its companion tool for many years, it has not been
superseded and is still used extensively, particularly to study
details of collisions between nuclear particles and atoms.
THE PHOTOGRAPHIC METHOD
The tracks of individual particles passing through matter can
also be observed in photographic emulsions, but the lengths of
path are so small that they must be observed under a microscope,
where they appear as a series of developed grains marking the
passage of the particle. This method of observation requires
practically no equipment but is tedious and of limited usefulness.
It is possible, however, to use the general blackening of a
photographic film as a measure of total exposure to radiation,
a procedure that has been used to supplement or to replace
electroscopes for safety control in many parts of the project.
THE OBSERVATION AND MEASUREMENT OF NEUTRONS
None of the methods we have described is directly applicable
to neutrons, but all of them are indirectly applicable since neu-
trons produce ions indirectly. This happens in two ways by
elastic collision and by nuclear reaction. As we have already
described, a fast neutron in passing through matter occasionally
approaches an atomic nucleus so closely as to impart to it a large
amount of momentum and energy according to the laws of
elastic collision. The nucleus thereby becomes a high-speed
charged particle which will produce ionization in an ionization
chamber, counter, or cloud chamber. But if the neutron has low
speed, e.g., thermal, the struck nucleus will not get enough
energy to cause ionization. If, on the other hand, the neutron is
absorbed and the resultant nucleus breaks up with the release of
energy, ionization will be produced. Thus, for the detection of
high-speed neutrons one has a choice between elastic collisions
and nuclear reaction, but for thermal speeds only nuclear reaction
will serve.
The reaction most commonly used is the <>B 10 (n, a)sLi 7 reaction
which releases about 2.5 Mev energy shared between the resultant
Observing Fast Particles 233
alpha particle and aLi 7 nucleus. This is ample to produce ioniza-
tion. This reaction is used by filling an ionization chamber or
proportional counter with boron trifluoride gas so that the reac-
tion occurs in the region where ionization is wanted; as an alterna-
tive the interior of the chamber or counter is lined with boron.
The ionization chamber then serves as an instrument to measure
overall neutron flux while the proportional counter records
numbers of individual neutrons.
One of the most valuable methods of measuring neutron densi-
ties by nuclear reactions depends on the production of artificial
radioactive nuclei. A foil known to be made radioactive by neu-
tron bombardment is inserted at a point where the neutron
intensity is wanted. After a given time it is removed and its
activity measured by an electroscope or counter. The degree of
activity that has been built up is then a measure of the number
of neutrons that have been absorbed. This method has the
obvious disadvantage that it does not give an instantaneous
response as do the ionization chamber and counter.
One of the most interesting methods developed on the project
is to use the fission of uranium as the nuclear reaction for neutron
detection. Furthermore, by separating the isotopes, fast and slow
neutrons can be differentiated.
Since the probability of a neutron reaction occurring is dif-
ferent for every reaction and for every neutron speed, difficulties
of translating counts or current measurements into numbers and
speeds of neutrons present are even greater than for other nuclear
particles. No one need be surprised if two able investigators give
different numbers for supposedly the same nuclear consant. It
is only by an intricate series of interlocking experiments carefully
compared and interpreted that the fundamental facts can be
untangled from experimental and instrumental variables.
APPENDIX 2. THE UNITS OF MASS, CHARGE
AND ENERGY
MASS
Since the proton and the neutron are the fundamental particles
out of which all nuclei are built, it would seem natural to use the
mass of one or the other of them as a unit of mass. The choice
would probably be the proton, which is the nucleus of a hydrogen
atom. There are good reasons, historical and otherwise, why
neither the proton nor the neutron was chosen. Instead, the mass
unit used in atomic and nuclear physics is one sixteenth of the
mass of the predominant oxygen isotope, O 16 , and is equal to
1.6603 X 10~ 24 gram. Expressed in terms of this unit, the mass
of the proton is 1.00758 and the mass of the neutron is 1.00893.
(Chemists usually use a very slightly different unit of mass.)
CHARGE
The unit of electric charge used in nuclear science is the posi-
tive charge of the proton. It is equal in magnitude but opposite
in sign to the charge on the electron and is therefore often called
the electronic charge. One electronic charge is 1.60 X 10~ 19
coulomb. It may be recalled that a current of one ampere flowing
for one second conveys a charge of one coulomb; i.e., one elec-
tronic charge equals 1.60 X 10~ 19 ampere second.
ENERGY
The energy unit used in nuclear physics is the electron volt,
which is defined as equal to the kinetic energy which a particle
carrying one electronic charge acquires in falling freely through
a potential drop of one volt. It is often convenient to use the
million-times greater unit: million electron volt (Mev).
234
Units of Mass, Charge, Energy
235
The relationships among the electron volt and other common
units of energy are in the following table:
CONVERSION TABLE FOR ENERGY UNITS
MULTIPLY
Mev
units
ergs
g. cal.
kw. hrs.
BY
1.07 X ID" 3
1.60 X 10~ 6
3.83 X 10~"
4.45 X 10- 20
9.31 X 10 2
1.49 X 10~ 3
3.56 X 10~"
4.15 X 10~ 17
6.71 X 10 2
6.24 X 10 5
2.39 X 10~ 8
2.78 X 10-"
2.81 X 10 10
2.62 X 10 13
4.18 X 10 7
1.16 X 10~ 6
2.41 X 10 16
2.25 X 10 19
3.60 X 10 13
8.60 X 10 5
TO OBTAIN
mass units
ergs
g. cal.
kw. hrs.
Mev
ergs
g. cal.
kw. hrs.
mass units
Mev
g. cal.
kw. hrs.
mass units
Mev
ergs
kw. hrs.
mass units
Mev
ergs
g. cal.
APPENDIX 3. DELATED NEUTRONS FROM
URANIUM FISSION
As was pointed out in Chapter VI, the control of a chain-
reacting pile is greatly facilitated by the fact that some of the
neutrons resulting from uranium fission are not emitted until
more than a second after fission occurs. It was therefore important
to study this effect experimentally. Such experiments were
described by Snell, Nedzel and Ibser in a report dated May 15,
1942 from which we quote as follows:
"The present experiment consists of two interrelated parts
one concerned with the decay curve, and one concerned with the
intensity of the delayed neutrons measured in terms of that of the
'instantaneous' fission neutrons.
THE DECAY CURVE OF THE DELAYED NEUTRONS
"The neutron source was the beryllium target of the University
of Chicago cyclotron struck by a beam of up to 20 pA of 8 Mev
deuterons. Near the target was placed a hollow shell made of
tinned iron and containing 106 Ibs. of UaOg. This was surrounded
by about 2" of paraffin. The interior of the shell was filled with
paraffin, except for an axial hole which accommodated a BF 3 -
filled proportional counter. The counter was connected through
an amplifier to a scaling circuit ('scale of 64') equipped with inter-
polating lights and a Cenco impulse counter. A tenth-second
timer, driven by a synchronous motor, and hundredth-second stop
watch were mounted on the panel of the sealer, close to the
interpolating lights and impulse counter. This group of dials and
lights was photographed at an appropriately varying rate by a
Sept camera which was actuated by hand. The result was a
record on movie film of times and counts, from which the decay
curves were plotted.
236
Delayed Neutrons from Uranium Fission 237
"The actual procedure was as follows: During bombardment
the stop watch was started and the timer was running con-
tinuously; the counter and amplifier were on, but the pulses
leaving the amplifier were grounded. The sealer was set at zero.
After a warning signal the cyclotron was shut off by one operator,
while another operator switched the output of the amplifier from
ground into the sealer, and started taking photographs. It was
easy to take the first photograph within half a second of turning
off the cyclotron. Sixty to a hundred photographs were taken
during a typical run. The necessity of using both a stop watch
and a timer arose from the fact that the hundredth-second pre-
cision of the stop watch was needed for the small time intervals
between photographs during the initial part of the run, but the
watch ran down and stopped before the counting was complete.
The timer then gave sufficient precision for the later time
intervals.
"Some forty runs were taken under varying experimental con-
ditions. Short activations of one or two seconds were given for
best resolution of the short periods. Long, intense bombardments
lasting 15-20 minutes, as close as possible to the target, were
made to make the long period activities show up with a maximum
intensity. Some 5-minute bombardments were made, keeping the
cyclotron beam as steady as possible, to study the relative satura-
tion intensities of the various activities; in these activations the
cyclotron beam was reduced to 1 or 2 /xA to prevent the initial
counting rate from becoming too high for a counter (300 per sec.
\\ as taken as a reasonable upper limit for reliable counting). Two
BF 3 counters were available, one having a thermal neutron cross
section of 2.66 sq. cm., and the other 0.43 sq. cm. After a strong
activation, we could follow the decay of the delayed neutrons
for some 13 minutes. Background counts (presumably chiefly
due to spontaneous fission neutrons) were taken and were sub-
tracted from the readings. They amounted to about 0.4 counts
per sec. for the large counter.
"A study of all the decay curves gives the following as a general
picture of the neutron-emitting activities present:
238 Appendix 3
TABLE 1
RELATIVE INITIAL INTENSITY
HALF-LIFE ACTIVATED TO SATURATION
57 3 sec. 0.135
24 2 sec. 1.0
7 sec. 1.2
2.5 sec. 1.2
"Any activity of period longer than 57 sec. failed to appear
even after the most intense bombardment we could give, lasting
20 minutes. The relative initial intensities given are the average
values obtained from three curves.
"These results give the following equation for the decay curve,
of the delayed neutrons after activation to saturation:
Activity = constant (1.2e-- 28 < + 1.2e-- 099 ' + 1.0e-- 029 <
-f 0.135e-- 012
where t is in seconds."
The second part of the experiment measured the total number
of neutrons emitted in the time interval 0.01 sec. to 2.0 min. after
the cyclotron was turned off. Assuming that all the delayed
neutrons observed were in the four groups measured in the first
part of the experiment, this second result indicated that 1.0 0.2
per cent of the neutrons emitted in uranium fission are delayed
by at least 0.01 sec. and that about 0.07 per cent are delayed by
as much as a minute. By designing the effective value of k, the
multiplication factor, for a typical operating pile to be only 1.01
with all the controls removed and the total variation in k from
one control rod to be 0.002, the number of delayed neutrons is
sufficient to allow easy control.
APPENDIX 4. THE FIRST SELF-SUSTAINING
CHAIN-REACTING PILE
In Chapter VI the construction and operation of the first self-
sustaining chain-reacting pile were described briefly. Though
details must still be withheld for security reasons, the following
paragraphs give a somewhat fuller description based on a report
by Fermi. This pile was erected by Fermi and his collaborators in
the fall of 1942.
DESCRIPTION OF THE PILE
The original plan called for an approximately spherical pile
with the best materials near the center. Actually control measure-
ments showed that the critical size had been reached before the
sphere was complete, and the construction was modified accord-
ingly. The final structure may be roughly described as an oblate
spheroid flattened at the top, i.e., like a door knob. It was desired
to have the uranium or uranium oxide lumps spaced in a cubic
lattice imbedded in graphite. Consequently, the graphite was
cut in bricks and built up in layers, alternate ones of which con-
tained lumps of uranium at the corners of squares. The critical
size was reached when the pile had been built to a height only
three quarters of that needed according to the most cautious
estimates. Consequently only one more layer was added. The
graphite used was chiefly from the National Carbon Company
and the Speer Graphite Company. The pile contained 12,400 Ibs.
of metal, part of which was supplied by Westinghouse, part by
Metal Hydrides, and part by Ames. Since there were many more
lattice points than lumps of metal, the remaining ones were filled
with pressed oxide lumps.
For purposes of control and experiment there were ten slots
passing completely through the pile. Three of those near the
239
240 Appendix 4
center were used for control and safety rods. Further to facilitate
experiment, particularly the removal of samples, one row of
graphite bricks carrying uranium and passing near the center of
the pile was arranged so that it could be pushed completely out
of the pile.
This whole graphite sphere was supported by a timber frame-
work resting on the floor of a squash cpurt under the West Stands
of Stagg Field.
PREDICTED PERFORMANCE OF THE PILE
The metal lattice at the center of the pile and the two other
major lattices making up the bulk of the rest of the pile had each
been studied separately in exponential experiments #18, #27, and
#29. These had given a multiplication factor of 1.07 for the metal
lattice and 1.04 and 1.03 for the oxide lattices, the difference in
the last two resulting from difference in the grade of graphite used.
It is to be remembered that these figures are multiplication factors
for lattices of infinite size. Therefore a prediction of the actual
effective multiplication factor k e ff for the pile as constructed
depended on the validity of the deduction of k from the ex-
ponential experiments, on a proper averaging for the different
lattices, and on a proper deduction of k eff from the average k for
infinite size. Although the original design of the pile had been
deliberately generous, its success when only partly completed
indicated that the values of the multiplication factors as calculated
from exponential experiments had been too low. The observed
effective multiplication factor of the part of the planned structure
actually built was about 1 .0006 when all neutron absorbers were
removed.
MEASUREMENTS PERFORMED DURING CONSTRUCTION
A series of measurements was made while the pile was being
assembled in order to be sure that the critical dimensions were
not reached inadvertently. These measurements served also to
check the neutron multiplication properties of the structure
First Self-Sustaining Pile
241
during assembly, making possible a prediction of where the
critical point would be reached.
In general, any detector of neutrons or gamma radiation can
be used for measuring the intensity of the reaction. Neutron
detectors are somewhat preferable since they give response more
F&ff
CRITICAL
LAYER
NUMBER OF COMPLETED LAYERS
FIGURE I
quickly and are not affected by fission-product radiations after
shut down. Actually both neutron detectors (boron trifluoride
counters) and gamma-ray ionization chambers were distributed
in and around the pile. Certain of the ionization chambers were
used to operate recording instruments and automatic safety
controls.
In the pile itself measurements were made with two types of
detector. A boron trifluoride counter was inserted in a slot about
242 Appendix 4
43" from the ground and its readings taken at frequent intervals.
In addition, an indium foil was irradiated every night in a posi-
tion as close as possible to the effective center of the pile, and its
induced activity was measured the following morning and com-
pared with the readings of the boron trifluoride counter.
The results of such measurements can be expressed in two ways.
Since the number of secondary neutrons produced by fission will
increase steadily as the pile is constructed, the activity A induced
in a standard indium foil at the center will increase steadily as
the number of layers of the pile is increased. Once the effective
multiplication factor is above one, A would theoretically increase
to infinity. Such an approach to infinity is hard to observe, so a
second way of expressing the results was used. Suppose the lattice
spacing and purity of materials of a graphite-uranium structure
are such that the multiplication factor would be exactly one if
the structure were a sphere of infinite radius. Then, for an actual
sphere of similar construction but finite radius, the activation of a
detector placed at the center would be proportional to the square
of the radius. It was possible to determine a corresponding
effective radius Rff for the real pile in each of its various stages.
It followed, therefore, that, if the factor k^ were precisely one on
the average for the lattice in the pile, the activity A of the de-
tector at the center should increase with increasing R e ff in such
a way that R 2 ef f/A remained constant, but, if k*, for the lattice
were greater than one, then as the pile size approached the
critical value, that is, as k^ff approached one, A should approach
infinity and therefore R 2 e ff/A approach zero. Therefore by extra-
polating a curve of R 2 e ff/A vs. size of the pile i.e., number of layers,
to where it cut the axis, it was possible to predict at what layer
k e ff would become one. Such a curve, shown in Fig. 1, indicated
at what layer the critical size would be reached. The less useful
but more direct and dramatic way of recording the results is
shown in Fig. 2, which shows the growth of the neutron activity
of the pile as layers were added.
During the construction, appreciably before reaching this
critical layer, some cadmium strips were inserted in suitable slots.
First Self -Sustaining Pile
243
4000
3000
LU
H
2000
1000
200
NUMBER OF COMPLETED LAYERS
FIGURE 2
244 Appendix 4
They were removed once every day with the proper precautions
in order to check the approach to the critical conditions. The
construction was carried in this way to the critical layer.
CONTROL
The reaction was controlled by inserting in the pile some strips
of neutron-absorbing material cadmium or boron steel. When
the pile was not in operation, several such cadmium strips were
inserted in a number of slots, bringing the effective multiplication
factor considerably below one. In fact, any one of the cadmium
strips alone was sufficient to bring the pile below the critical
condition. Besides cadmium strips that could be used for manual
operation of the pile, two safety rods and one automatic control
rod were provided. The automatic control rod was operated by
two electric motors responding to an ionization chamber and
amplifying system so that, if the intensity of the reaction increased
above the desired level, the rod was pushed in, and vice versa.
OPERATION OF THE PILE
To operate the pile all but one of the cadmium strips were
taken out. The remaining one was then slowly pulled out. As
the critical conditions were approached, the intensity of the
neutrons emitted by the pile began to increase rapidly. It should
be noticed, however, that, when this last strip of cadmium was so
far inside the pile that the effective multiplication factor was just
below one, it took a rather long time for the intensity to reach
the saturation value. Similarly, if the cadmium strip was just
far enough out to make k e ff greater than one, the intensity rose
at a rather slow rate. For example, if one rod is only 1 cm. out
from the critical position, the "relaxation time," i.e., the time
for the intensity to double, is about four hours. These long
"relaxation times" were the result of the small percentage of
delayed neutrons which have been discussed in Appendix 3, and
make it relatively easy to keep the pile operating at a constant
level of intensity.
First Self-sustaining Pile 245
The pile was first operated on December 2, 1 942 to a maximum
energy production of about J^ watt. On December 12th the
intensity was run up to about 200 watts, but it was not felt safe
to go higher because of the danger of the radiation to personnel
in and around the building. During this high intensity run,
measurements were made of radiation intensity beside the pile,
in the building, and on the sidewalk outside.
APPENDIX 5. SAMPLE LIST OF REPORTS
Presented below is a list of titles of representative reports pre-
pared in the Metallurgical Laboratory of the University of Chi-
cago in 1942.
A Table for Calculating the Percentage Loss Due to the
Presence of Impurities in Alloy
Concerning the Radium-Beryllium Neutron Sources
Preliminary Estimates of the Radiations from Fission
Products
Background of Natural Neutrons in Multiplying Pile
Absorption Cross Sections for Rn plus Be Fast Neutrons
On Mechanical Stresses Produced by Temperature Gra-
dients in Rods and Spheres
Effect of Geometry on Resonance Absorption of Neutrons by
Uranium
Protection against Radiations
Planning Experiments on Liquid Cooling
Report on the Possibility of Purifying Uranium by Carbonyl
Formation and Decomposition
On the Radioactivity of Cooling Helium
Estimation of Stability of Ether under Various Conditions of
Irradiation
Uranium Poisoning
Transuranic and Fission Product Activities
Chemical Effects of Radiation on Air Surrounding the Pile
An Estimate of the Chemical Effects of Radiation on the
Cooling Water in the Pile
The Extraction Method of Purification of Uranyl Nitrate
The Diffusion of Fission Products from Cast Metal at 600C
and 1000C
246
APPENDIX 6. WAR DEPARTMENT RELEASE
ON NEW MEXICO TEST, JULY 16, 1945
"Mankind's successful transition to a new age, the Atomic Age,
was ushered in July 16, 1945, before the eyes of a tense group of
renowned scientists and military men gathered in the desertlands
of New Mexico to witness the first end results of their $2,000,000,-
000 effort. Here in a remote section of the Alamogordo Air Base
120 miles southeast of Albuquerque the first man-made atomic
explosion, the outstanding achievement of nuclear science, was
achieved at 5: 30 a.m. of that day. Darkening heavens, pouring
forth ram and lightning immediately up to the zero hour, height-
ened the drama.
"Mounted on a steel tower, a revolutionary weapon destined
to change war as we know it, or which may even be the instru-
mentality to end all wars, was set off with an impact which
signalized man's entrance into a new physical world. Success was
greater than the most ambitious estimates. A small amount of
matter, the product of a chain of huge specially constructed
industrial plants, was made to release the energy of the universe
locked up within the atom from the beginning of time. A fabulous
achievement had been reached. Speculative theory, barely estab-
lished in pre-war laboratories, had been projected into practicality.
"This phase of the Atomic Bomb Project, which is headed by
Major General Leslie R. Groves, was under the direction of Dr.
J. R. Oppenheimer, theoretical physicist of the University of
California. He is to be credited with achieving the implementa-
tion of atomic energy for military purposes.
"Tension before the actual detonation was at a tremendous
pitch. Failure was an ever-present possibility. Too great a success,
envisioned by some of those present, might have meant an uncon-
trollable, unusable weapon.
"Final assembly of the atomic bomb began on the night of
247
248 Appendix 6
July 12 in an old ranch house. As various component assemblies
arrived from distant points, tension among the scientists rose to
an increasing pitch. Coolest of all was the man charged with the
actual assembly of the vital core, Dr. R. F. Bacher, in normal
times a professor at Cornell University.
"The entire cost of the project, representing the erection of
whole cities and radically new plants spread over many miles of
countryside, plus unprecedented experimentation, was repre-
sented in the pilot bomb and its parts. Here was the focal point
of the venture. No other country in the world had been capable
of such an outlay in brains and technical effort.
"The full significance of these closing moments before the final
factual test was not lost on these men of science. They fully knew
their position as pioneers into another age. They also knew that
one false move would blast them and their entire effort into
eternity. Before the assembly started a receipt for the vital matter
was signed by Brigadier General Thomas F. Farrell, General
Groves' deputy. This signalized the formal transfer of the irre-
placeable material from the scientists to the Army.
"During final preliminary assembly, a bad few minutes devel-
oped when the assembly of an important section of the bomb was
delayed. The entire unit was machine- tooled to the finest meas-
urement. The insertion was partially completed when it appar-
ently wedged tightly and would go no farther. Dr. Bacher,
however, was undismayed and reassured the group that time
would solve the problem. In three minutes' time, Dr. Bacher's
statement was verified and basic assembly was completed without
further incident.
"Specialty teams, comprised of the top men on specific phases
of science, all of which were bound up in the whole, took over
their specialized parts of the assembly. In each group was central-
ized months and even years of channelized endeavor.
"On Saturday, July 14, the unit which was to determine the
success or failure of the entire project was elevated to the top of
the steel tower. All that day and the next, the job of preparation
went on. In addition to the apparatus necessary to cause the
New Mexico Test, July 16, 1945 249
detonation, complete instrumentation to determine the pulse
beat and all reactions of the bomb was rigged on the tower.
"The ominous weather which had dogged the assembly of the
bomb had a very sobering affect on the assembled experts whose
work was accomplished amid lightning flashes and peals of
thunder. The weather, unusual and upsetting, blocked out aerial
observation of the test. It even held up the actual explosion
scheduled at 4:00 a.m. for an hour and a half. For many months
the approximate date and time had been set and had been one
of the high-level secrets of the best kept secret of the entire war.
"Nearest observation point was set up 10,000 yards south of
the tower where in a timber and earth shelter the controls for the
test were located. At a point 17,000 yards from the tower at a
point which would give the best observation the key figures in
the atomic bomb project took their posts. These included General
Groves, Dr. Vannevar Bush, head of the Office of Scientific
Research and Development and Dr. James B. Conant, president
of Harvard University.
"Actual detonation was in charge of Dr. K. T. Bainbridge of
Massachusetts Institute of Technology. He and Lieutenant Bush,
in charge of the Military Police Detachment, were the last men
to inspect the tower with its cosmic bomb.
"At three o'clock in the morning the party moved forward to
the control station. General Groves and Dr. Oppenheimer con-
sulted with the weathermen. The decision was made to go ahead
with the test despite the lack of assurance of favorable weather.
The time was set for 5:30 a.m.
"General Groves rejoined Dr. Conant and Dr. Bush, and just
before the test time they joined the many scientists gathered at
the Base Camp. Here all present were ordered to lie on the
ground, face downward, heads away from the blast direction.
"Tension reached a tremendous pitch in the control room as
the deadline approached. The several observation points in the
area were tied in to the control room by radio and with twenty
minutes to go, Dr. S. K. Allison of Chicago University took over
the radio net and made periodic time announcements.
250 Appendix 6
"The time signals, 'minus 20 minutes, minus fifteen minutes,'
and on and on increased the tension to the breaking point as the
group in the control room which included Dr. Oppenheimer and
General Farrell held their breaths, all praying with the intensity
of the moment which will live forever with each man who was
there. At 'minus 45 seconds/ robot mechanism took over and
from that point on the whole great complicated mass of intricate
mechanism was in operation without human control. Stationed
at a reserve switch, however, was a soldier scientist ready to
attempt to stop the explosion should the order be issued. The
order never came.
"At the appointed time there was a blinding flash lighting up
the whole area brighter than the brightest daylight. A mountain
range three miles from the observation point stood out in bold
relief. Then came a tremendous sustained roar and a heavy
pressure wave which knocked down two men outside the control
center. Immediately thereafter, a huge multi-colored surging
cloud boiled to an altitude of over 40,000 feet. Clouds in its path
disappeared. Soon the shifting substratosphere winds dispersed
the now grey mass.
"The test was over, the project a success.
"The steel tower had been entirely vaporized. Where the tower
had stood, there was a huge sloping crater. Dazed but relieved at
the success of their tests, the scientists promptly marshalled their
forces to estimate the strength of America's new weapon. To
examine the nature of the crater, specially equipped tanks were
wheeled into the area, one of which carried Dr. Enrico Fermi,
noted nuclear scientist. Answer to their findings rests in the
destruction effected in Japan today in the first military use of the
atomic bomb.
"Had it not been for the desolated area where the test was held
and for the cooperation of the press in the area, it is certain that
the test itself would have attracted far-reaching attention. As it
was, many people in that area are still discussing the effect of the
smash. A significant aspect, recorded by the press, was the experi-
ence of a blind girl near Albuquerque many miles from the
New Mexico Test, July 76, 7945 251
scene, who, when the flash of the test lighted the sky before the
explosion could be heard, exclaimed, 'What was that?'
"Interviews of General Groves and General Farrell give the
following on-the-scene versions of the test. General Groves said:
'My impressions of the night's high points follow: After about an
hour's sleep I got up at 0100 and from that time on until about
five I was with Dr. Oppenheimer constantly. Naturally he was
tense, although his mind was working at its usual extraordinary
efficiency. I attempted to shield him from the evident concern
shown by many of his assistants who were disturbed by the un-
certain weather conditions. By 0330 we decided that we could
probably fire at 0530. By 0400 the rain had stopped but the sky
was heavily overcast. Our decision became firmer as time
went on.
" ' During most of these hours the two of us journeyed from the
control house out into the darkness to look at the stars and to
assure each other that the one or two visible stars were becoming
brighter. At 0510 I left Dr. Oppenheimer and returned to the
main observation point which was 17,000 yards from the point
of explosion. In accordance with our orders I found all personnel
not otherwise occupied massed on a bit of high ground.
" 'Two minutes before the scheduled firing time, all persons lay
face down with their feet pointing towards the explosion. As the
remaining time was called from the loud speaker from the 10,000-
yard control station there was complete awesome silence. Dr.
Conant said he had never imagined seconds could be so long.
Most of the individuals in accordance with orders shielded their
eyes in one way or another.
" 'First came the burst of light of a brilliance beyond any com-
parison. We all rolled over and looked through dark glasses at
the ball of fire. About forty seconds later came the shock wave
followed by the sound, neither of which seemed startling after
our complete astonishment at the extraordinary lighting intensity.
" 'A massive cloud was formed which surged and billowed
upward with tremendous power, reaching the substratosphere in
about five minutes.
252 Appendix 6
" 'Two supplementary explosions of minor effect other than the
lighting occurred in the cloud shortly after the main explosion.
" 'The cloud traveled to a great height first in the form of a
ball, then mushroomed, then changed into a long trailing chim-
ney-shaped column and finally was sent in several directions by
the variable winds at the different elevations.
" 'Dr. Conant reached over and we shook hands in mutual con-
gratulations. Dr. Bush, who was on the other side of me, did
likewise. The feeling of the entire assembly, even the uninitiated,
was of profound awe. Drs. Conant and Bush and myself were
struck by an even stronger feeling that the faith of those who had
been responsible for the initiation and the carrying on of this
Herculean project had been justified."
General FarrelPs impressions are: "The scene inside the shelter
was dramatic beyond words. In and around the shelter were some
twenty odd people concerned with last-minute arrangements.
Included were Dr. Oppenheimer, the Director who had borne
the great scientific burden of developing the weapon from the
raw materials made in Tennessee and Washington, and a dozen
of his key assistants, Dr. Kistiakowsky, Dr. Bainbridge, who super-
vised all the detailed arrangements for the test; the weather
expert, and several others. Besides those, there were a handful
of soldiers, two or three Army officers and one Naval Officer.
The shelter was filled with a great variety of instruments and
radios.
" 'For some hectic two hours preceding the blast, General
Groves stayed with the Director. Twenty minutes before the
zero hour, General Groves left for his station at the base camp,
first because it provided a better observation point and second,
because of our rule that he and I must not be together in situa-
tions where there is an element of danger which existed at both
points.
" 'Just after General Groves left, announcements began to be
broadcast of the interval remaining before the blast to the other
groups participating in and observing the test. As the time interval
grew smaller and changed from minutes to seconds, the tension
New Mexico Test, July 16, 1945 253
increased by leaps and bounds. Everyone in that room knew the
awful potentialities of the thing that they thought was about to
happen. The scientists felt that their figuring must be right and
that the bomb had to go off but there was in everyone's mind a
strong measure of doubt.
" 'We were reaching into the unknown and we did not know
what might come of it. It can safely be said that most of those
present were praying and praying harder than they had ever
prayed before. If the shot were successful, it was a justification
of the several years of intensive effort of tens of thousands of
people statesmen, scientists, engineers, manufacturers, soldiers,
and many others in every walk of life.
"'In that brief instant in the remote New Mexico desert, the
tremendous effort of the brains and brawn of all these people
came suddenly and startlingly to the fullest fruition. Dr. Oppen-
heimer, on whom had rested a very heavy burden, grew tenser
as the last seconds ticked off. He scarcely breathed. He held on
to a post to steady himself. For the last few seconds, he stared
directly ahead and then when the announcer shouted "Now!"
and there came this tremendous burst of light followed shortly
thereafter by the deep growling roar of the explosion, his face
relaxed into an expression of tremendous relief. Several of the
observers standing back of the shelter to watch the lighting effects
were knocked flat by the blast.
" 'The tension in the room let up and all started congratulating
each other. Everyone sensed 'This is it!'. No matter what might
happen now all knew that the impossible scientific job had been
done. Atomic fission would no longer be hidden in the cloisters
of the theoretical physicists' dreams. It was almost full grown at
birth. It was a great new force to be used for good or for evil.
There was a feeling in that shelter that those concerned with its
nativity should dedicate their lives to the mission that it would
always be used for good and never for evil.
" 'Dr. Kistiakowsky threw his arms around Dr. Oppenheimer
and embraced him with shouts of glee. Others were equally
enthusiastic. All the pent-up emotions were released in those few
254 Appendix 6
minutes and all seemed to sense immediately that the explosion
had far exceeded the most optimistic expectations and wildest
hopes of the scientists. All seemed to feel that they had been
present at the birth of a new age The Age of Atomic Energy
and felt their profound responsibility to help in guiding into
right channels the tremendous forces which had been unlocked
for the first time in history.
"'As to the present war, there was a feeling that no matter
what else might happen, we now had the means to insure its
speedy conclusion and save thousands of American lives. As to
the future, there had been brought into being something big
and something new that would prove to be immeasurably more
important than the discovery of electricity or any of the other
great discoveries which have so affected our existence.
" 'The effects could well be called unprecedented, magnificent,
beautiful, stupendous and terrifying. No man-made phenomenon
of such tremendous power had ever occurred before. The lighting
effects beggared description. The whole country was lighted by a
searing light with the intensity many times that of the midday
sun. It was golden, purple, violet, gray and blue. It lighted every
peak, crevasse and ridge of the nearby mountain range with a
clarity and beauty that cannot be described but must be seen to
be imagined. It was that beauty the great poets dream about but
describe most poorly and inadequately. Thirty seconds after, the
explosion came first, the air blast pressing hard against the people
and things, to be followed almost immediately by the strong,
sustained, awesome roar which warned of doomsday and made
us feel that we puny things were blasphemous to dare tamper with
the forces heretofore reserved to the Almighty. Words are inade-
quate tools for the job of acquainting those not present with the
physical, mental and psychological effects. It had to be witnessed
to be realized.' "
INDEX
INDEX OF PERSONS
References are to chapter and paragraph except references to appendices which
are shown as A-7 etc.
Abelson, P. H., 4:36, 6:34, 11:38,
11:39
Adamson, Col. K. F., 3:4, 3:5, 3:6
Adler, E., 10:26
Aebersold, P. C., 11:6
Akers, W. A., 5:14
Allison, S. K., 3:14, 4:18, 4:19,
4:20, 4:43, 5:2, 5:4, 5:18, 6:5,
6:22, 7:18, 12:23, A-6
Anderson, C. D., 1 :21
Anderson, H. L., 4:2, 4:13, 6:31,
8:27
Arnold, H. R., 9:40
Arnold, J. H., 10:24
Aston, F. W., 1:35, 9:15
Bacher, R. F., 12:23, A-6
Backus, J. G., 11:6
Baker, A. L., 10:24
Barr, F. T., 9:41
Baxter, J. W., 11:34
Beams, J. W., 3:3, 3:9, 3:14, 5:2,
5:4, 5:12, 5:18, 9:18
Beans, H. T., 4:44
Becker, H., 1:19
Becquerel, H., 1:6, 1:7
Benedict, M., 9:32, 10:3
Bethe, H., 1:51, 12:23, 12:24
Bohr, N., 1:53,1:57,3:2,12:23
Boorse, H. A., 10:28
Booth, E. T., 10:1, 10:2, 10:7,
10:26
Bothe, W., 1:19
Bowen, Adm. H. G., 3:6, 3:7
Boyd, G. E., 6:33
Breit, G., 3:3, 3:8, 3:9, 3:14, 4:22,
5:2, 5:4, 5:18, 6:37, 6:38, 12:2
Brewer, A. K., 9:31
Brewster, O. C., 10:28
Brickwedde, F. G., 1:21
Briggs,L.J,3:3,3:4 5 3:5,3:6,3:7,
3:8, 3:9, 3:14, 3:15, 3:16, 4:3,
4:36, 5:2, 5:4, 5:6, 5:8, 5:14,
5:17, 10:2
Brobeck, F. A., 11:6
Buckley, O. E., 3:16
Bush, V., 3:9, 3:15, 3:16, 3:17,
3:18, 3:19, 3:21, 3:22, 5:2, 5:4,
5:8, 5:9, 5:13, 5:14, 5:17, 5:20,
5:21, 5:22, 5:25, 5:26, 5:27,
5:28, 5:33, A-6
Cady, G. H., 10:29
Cantril, S. T., 7:48
Carpenter, W. S., Jr., 7:5
Chadwick, J., 1:19, 3:17, 4:47,
12:23
Chapman, S., 9:21
Chilton, T. H., 3:14
Chubb, L. W., 3:16, 5:5
Clark, W. M., 3:3
Clusius, H., 9:22
Cockcroft, J. D., 1:38, 1:40, 3:17
Cohen, K., 4:34, 9:32, 10:3, 10:7
Cole, Dr., 8:70
255
256
Index
Compton, A. H., 3:15, 3:16, 3:19,
4:18, 5:4, 5:7, 5:8, 5:12, 5:14,
5:17, 5:21, 5:31, 6:2, 6:8, 6:22,
7:1, 7:48
Conant, J. B., 3:16, 3:18, 3:19,
5:4, 5:6, 5:8, 5:9, 5:10, 5:14,
5:17, 5:21, 5:25, 5:27, 5:31,
5:33, 12:5, A-6
Condon, E. U., 3:14, 5:2, 5:4,
5:18,11:6
Cooksey, D., 11:6
Coolidge, W. D., 3:15, 3:16
Cooper, C. M., 6:5, 7:48
Coryell, C. D., 6:33
Creutz, E. C., 4:12, 6:36
Crist, R. H., 9:40
Curie, I., 1:19,1:27
Curie, M., 1:6
Curie, P., 1:6
Curme, G. O., Jr., 5:5
Currie, L. M., 10:23
Daniels, F., 7:48
Dempster, A. J., 7:48, 9:29
Dickel, G., 9:22
Doan, R. L., 6:5, 6:8, 7:48
Dunning, J. R., 1:53, 10:1, 10:2,
10:23, 10:41
Eckart, C. H., 3:14, 4:22
Einstein, A., 1:4, 1:9, 1:39, 1:53,
3:4
Eisenhart, L. P., 3:3
Emmett, P. H., 10:23
Enskog, D., 9:21
Farrell, Gen. T. F., 5:32, A-6
Feld, B., 4:13
Fermi, E., 1:28, 1:52, 2:1, 2:10,
2:11, 3:1, 3:4, 3:6, 3:8, 3:14,
4:2, 4:13, 4:17, 4:22, 5:12, 6:5,
6:22, 6:31, 7:48, 8:27, 12:23,
A-4, A-6
Feynman, R. P., 9:32
Finkelstein, T., 11:6
Fletcher, H., 3:3
Fowler, R. H., 3:17, 4:38
Franck,J., 6:5, 7:48
Fred, E. B., 3:3
Frisch, O. R., 1:53, 1:54
Gary, T. C., 6:6
Gherardi, B., 3:15
Greenewalt, C. H., 6:6, 7:50
Grosse, A. von, 4:39, 9:40, 10:1,
10:2
Groves, Gen. L. R., 5:24, 5:25,
5:27, 5:28, 5:31, 5:32, 5:33,
7:4, 7:5, 7:12, 8:50, 10:34,
11:18, 12:2, 12:5, 13:5, 13:6,
A-6
Gunn, R., 3:9, 4:36, 5:2
Hahn, O., 1:53, 1:54
Halban, H., 5:14
Hamilton, Dr., 8:70
Helmholz, A. C., 11:6
Henne, A. L., 10:29
Heydenburg, N. P., 4:29
Hertz, G., 9:15
Hilberry, N., 6:8, 6:20, 6:33,
6:45, 7:48
Hoffman, J. I., 6:12
Hogness, T., 6:5, 7:48
Hoover, Comm. G. C., 3:4, 3:5,
3:6
Jenkins, F. A., 11:6
Jewett, F. B., 3:15
Johns, I. B., 6:33
Johnson, C. A., 10:26
Johnson, W. C., 6:5, 7:48
Joliot, F., 1:19, 1:27, 1:53, 3:2
Kaplan, L, 9:32
Keith, P. C., 5:5, 10:24
Index
257
Kennedy, J. W., 4:24, 6:33, 6:34,
12:23, 12:48
Kingdon, K. H., 11:6
Kistiakowski, G. B., 3:16, 12:23,
A-6
Langmuir, I., 11:6
Lauritsen, C. C., I
Lawrence, E. O., 3:15, 3:16, 3:17,
3:19,4:24,4:25,4:30,4:31,5:4,
5:7, 5:8, 5:17, 5:21, 5:31, 6:2,
11:1, 11:2, 11:4, 11:5, 11:6,
11:26, 11:29, 11:45
Leverett, M. C., 6:5, 7:15
Lewis, W. K., 3:16, 5:5, 6:6, 13:5
Libby, W. F., 10:2
McBee, E. T., 10:29
Mack, E., 10:23
MacKenzie, K. R., 11:6
McMillan, E., 6:34
Manley,J. H., 6:39
Marshall, Gen. G. C., 5:9, 5:21,
7:5
Marshall, Col. J. C., 5:23, 5:28,
10:34
Matthias, Col. F. T., 5:32
Meitner, L., 1:53, 1:54
Miles, J. B., 7:50
Miller, W. T., 10:29
Mills, Adm. E. W., 13:5
Mitchell, A. C. G., 4:11
Mohler, F. L., 3:5
Moore, T. W., 6:5, 7:15
Moses, Gen. R. G., 5:26
Mulliken, R. S., 3:16
Murphree, E. V., 5:4, 5:5, 5:8,
5:17, 6:6, 6:8, 9:41, 10:2
Murphy, G. M., 1:21, 10:23
Nichols, Col. K. D., 5:32, 10:34
Nier, A. O., 4:29, 10:1, 10:29, 11 :4
Nix, F. C., 10:26
Norris, E. O., 10:26
Oliphant, M. L. E., 3:19, 11:6,
11:34
Oppenheimer, F., 11:6
Oppenheimer, J. R., 5:31, 6:37,
6:39, 11:6, 12:2, 12:5, 12:23,
12:50, A-6
Parkins, W. E., 11:6
Parsons, Capt. W. S., 12:23
Paxton, H. C., 10:7, 10:26
Pegram, G. B., 1:53, 3:3, 3:4, 3:6,
3:8, 3:9, 3:14, 3:20, 3:21, 3:22,
4:2, 4:3, 4:5, 4:6, 4:46, 5:2, 5:4,
10:2
Peierls, R., 10:4
Perlman, L, 6:33
Peters, B., 11:6
Purnell, Adm. W. R., 5:25, 5:26,
5:27, 5:33
Rayleigh, Lord, 9:14
Reynolds, W. B., 11:6
Richardson, J. R., 11:6
Roberts, R. B., 3:5
Rodden, C.J., 4:44, 6:16
Roosevelt, President F. D., 3:4,
3:5, 3:6, 3:7, 3:9, 3:22, 3:23,
5:25, 5:33, 7:5
Rosen, R., 10:29
Ruhoff, Col., 6:19
Rutherford, E., 1:6, 1:17, 1:38,
A-l
Sachs, A., 3:4, 3:5, 3:6, 3:7
Seaborg, G. T., 4:24, 6:33, 6:34
Segre, E., 4:24
Serber, R., 6:39
Simon, F., 5:14, 10:4
Slack, F. G., 10:2, 10:26
258
Index
Slade, C. B., 10:7, 10:26
Slater, J. C., 3:15
Slepian,J., 11:2,11:6, 11:29
Sloan, D. H., 11:6
Smith, C. S., 12:23, 12:48
Smith, L.S., 11:6, 11:29
Smyth, H. D., 3:14, 4:12, 4:27,
4:28, 4:31, 5:2, 5:4, 5:18, 7:48,
8:56, 11:1, 11:24, 11:29, 13:5
Snell, A-3
Solberg, Capt. T. A., 13:5
Somervell, Gen. B., 5:25
Spedding, F. H., 6:5, 6:15, 6:17,
6:33
Stearns, J. C., 7:48
Squires, A. M., 9:32
Stewart, I., 5:17
Stimson, H. L., 5:9, 5:21, 5:25, 7:5
Stone, Dr. R. S., 6:5, 7:48, 8:70
Strassmann, F., 1:53, 1:54
Steyer, Gen. W. D., 5:21, 5:22,
5:25, 5:27, 5:33
Swearingen, J. S., 10:28
Szilard, L., 2:10, 2:11, 3:1, 3:4,
3:5, 3:6, 3:8, 3:14, 4:2
Taylor, H. S., 9:40, 10:23, 10:26
Teller, E., 3:1, 3:5, 3:9, 6:39
Thiele, E. W., 6:33
Thomas, C. H., 7:48
Thomson, G. P., 3:18
Thomson, J. J., 1 :35
Thornton, R. L., 11:6
Tolman, R. C., 5:31, 13:5
Truman, President H. S., 5:34
Turner, L. A., 1:53, 1:58
Tuve, M. A., 3:8, 3:9, 4:29
Urey, H. C., 1:21, 3:3, 3:7, 3:8,
3:9, 3:12, 3:14, 3:20, 3:21,
3:22, 3:23, 4:32, 4:38, 4:46,
5:2, 5:4, 5:7, 5:8, 5:12, 5:17,
5:21, 5:31, 6:2, 6:33, 9:18,
10:2, 10:23, 10:41
VanVleck,J. H., 3:15
Vernon, H. C., 7:48, 8:56
Wahl, A. C., 4:24, 6:33, 6:34
Wallace, H. A., 3:22, 5:9, 5:21
Walton, E. T. S., 1:38, 1:40
Warren, Col. S. L., 5:32
Watson, Gen. E. M., see Roosevelt,
President F. D.
Watson, W. W., 7:48
Watts, G. W., 10:28
Weil, G., 4:13, 8:27
Weisskopf, V. F., 3:1
Weed, L. H., 3:3
Wensel, H. T., 5:2, 5:4, 5:17
Wever, E. G., 3:3
Wheeler, J. A., 1:53, 1:57, 3:14,
4:12,4:22,6:5,6:33,7:50
Whitaker, M. D., 7:48, 7:50, 8:34
Wigner, E., 3:1, 3:3, 3:4, 3:5, 3:6,
3:8, 3:9, 4:12, 4:22, 4:27, 4:28,
6:5, 6:33, 7:15, 7:48, 8:56
Wilhelm, H. A., 6:33
Williams, R., 6:6, 7:50
Wilson, C. T. R., A-l
Wilson, R. R., 4:12, 11:24, 12:23
Wilson, V. C., 6:31
Zinn, W. H., 4:13, 6:31, 8:27, 8:61
INDEX OF CHIEF SUBJECTS
Actinium, 1:15, 1:16
Advisory Committee on Uranium,
3:4, 3:5, 3:8, 3:9. See also Urani-
um Committee (NDRC), ND-
RC Section S-l, OSRD Section
S-l, OSRD Section S-l Execu-
tive Committee
Allis-Chalmers Manufacturing
Company, 11:19, 11:45
Alpha particle, 1:33, 2:32, 4:24;
bombardment by 1:17, 1:19,
1:40; product of nuclear disin-
tegration; 1:9, 1:15, 1:16, 1:38,
1:51, A-l
Aluminum, 7:22
Argonne Laboratory, 8:27, 8:28,
8:59
Atomic number, 1:12, 1:15
Atomic structure, 1:6, 1:10, 1:11
Bakelite Corp., 10:26
Barium, 1:54
Bell Telephone Laboratories, Inc.,
10:26
Beryllium, 1:19, 1:40, 1:45, 2:10,
2:13, 2:29, 2:36, 4:10, 4:14,
4:18, 4:19, 4:43, 6:8, 6:22
Beta particles, 1:9, 1:15, 1:16,
2:32, 4:24, 6:34, 8:17, A-l
Binding energies, nuclear, l:31ff.,
1:36, 1:54
Bomb, atomic, 2:1, 2:3, 2:14,
2:16, 2:23, 2:34, 2:35, 2:36, 3:5,
4:25, 4:48, 4:49, 5:2, 5:10, 5:16,
6:1, 6:32, 6:37, 11:28, 11:42,
12:1, 13:1, A-6
Boron, 1:19, 1:40, 2:10, 4:42,
6:11, A-l, A-3
Cadmium, 3:11, 4:6, 7:27, 8:8, A-4
California Institute of Technology,
1:21
California, University of, 1:53,
3:12, 4:7, 4:24, 4:25, 6:33,
6:34, 6:38, 11:2, 11:6, 11:12,
11:19,11:23,11:44, 11:45, 12:7,
13:1
Canadian Radium and Uranium
Corp., 6:10
Carbide and Carbon Chemicals
Corp., 10:2, 10:25, 10:11
Carbon, 1:51, 2:10, 2:13, 3:5, 3:6,
3:8, 4:2
Carnegie Institute of Washington,
1:53,3:12,4:29,6:38
Centrifugal isotope separation,
4:32, 4:37, 4:49, 5:6, 5:15,
9:18,9:36,9:42, 11:16
Chain reaction, 1:50, 1:51, 1:56,
2:3ff., 2:23, 2:31, 2:32, 2:33,
2:34, 2:36, 4:1, 4:2ff., 4:47,
5:7, 6:6, 6:21ff., 8:3ff., 12:8
Chemical analysis, 4:44, 7:34
Chemical exchange method of
isotope separation, 4:39, 9:24,
9:39
Chemical problems, 7:39, 11:35,
A-5
Chicago, University of, 3:12, 4:7,
4:11, 4:18, 4:20, 5:12, 6:27,
6:33,6:38,7:8,7:9,8:50, 11:41,
259
260
Index
Chicago, University of, 12:32,
13:1. See also Metallurgical Lab-
oratory, 29
Clinton Engineer Works, 7:8,
10:33, 10:42, 11:44, 11:45, 13:1
Clinton, Tenn., 7:1, 8:34, 6:44
Cloud chamber, 1:7, 12:35, A-l
Columbia University, 1:53, 3:6,
3:7, 3:10, 3:12, 4:2, 4:8, 4:9,
4:13, 4:17, 4:22, 4:32, 4:38,
4:39, 4:40, 4:41, 5:12, 8:56,
10:1, 10:23, 10:26, 10:39, 11:41,
13:1
Consolidated Mining and Smelting
Co., 9:41
Controls, 7:27, 8:8
Cornell University, 3:12, 6:28
Cooling, 6:43, 7:13, 7:15, 7:16,
7:18, 7:26, 8:8, A-5
Corrosion, 7:21, 7:25, 7:38, 10:17,
8:52
Counter, Geiger-Muller, 1:7, 4:6,
12:35, A-l
Counter, proportional, A-l, A-3
Cross Section, nuclear, 1 :45ff.,
4:6, 12:12, A-5. See also nuclei
involved
Cyclotron, 1:40, 4:10
Danger coefficients, 8:28
Decontamination, 8:23
Detection of nuclear particles, 4:6,
4:13, 8:39, 8:67, 12:35, A-l.
See also Cloud Chamber, Coun-
ter, Electroscope, lonization
chamber
Deuterium, see Heavy water
Deuteron, 1:21, 1:40, 6:34
DSM Project, 5:23, 7:4
du Pont, E. I., de Nemours and
Co., Inc., 6:13, 6:18, 6:40, 7:4,
7:5, 7:6, 8:50, 9:40, 10:29,
11:19, 7:50ff.
Electrolytic method of isotope
separation, 9:25
Electromagnetic methods of iso-
tope separation, 4:30, 5:7, 5:15,
9:28, ll:lff. Calutron, 11:2,
11:10, 11:20. Ion source, 9:30,
11:4, 11:5, 11:25. Isotron, 9:31,
11:2, 11:24. Magnetron (ion
centrifuge), 9:31, 11:2, 11:29.
Space charge effects 9:30, 11:5,
11:8
Electron, 1:11, 1:12, 1:23
Electroscope, 1:7, 12:35, A-l
Energy, conservation of, 1:2
Enrichment factor, 9:8, 9:14, 9:20,
9:30, 10:6, 10:7, 11:14, 11:21
Equilibrium, radioactive, 8:18
Fast fission effect, 8:9
Feasibility Report, 6:6, 6:39, 6:43,
6:45
Fission, 1:3, 1:53, 1:55, 1:57, 2:3,
2:14, 2:19, 2:21, 2:35, 6:38,
12:16. See also Neutron, Plu-
tonium, Uranium
Fission Products, 1:53, 1:56, 1:57,
2:10, 2:32, 4:6, 7:29, 7:35, 8:17,
A-5
Fractional distillation method of
isotope separation, 9:16, 9:38
Gamma rays, 1:9, 1:15, 1:19, 2:32,
8:5, 8:17, A-l
Gaseous diffusion method of iso-
tope separation, 4:32, 4:37,
4:47, 4:49, 5:6, 5:12, 5:14,
9:14, 10:lff., 10:14, 10:26, 11:16,
11:32
General Electric Co., 11:19, 11:45
George Washington University,
1:53
Graphite, 2:9, 2:19, 2:30, 2:31,
2:36, 3:5, 3:7, 3:11, 4:2, 4:4,
Index
261
Graphite, 4:6, 4:13, 4:42, 4:47,
5:14, 6:8, 6:28, 6:29
Half Life, 1:16, 6:28, 8:17. See also
Nuclei involved
Hanford Engineer Works, 7:8,
7:12, 8:50, 10:78, 13:1
Harshaw Chemical Co., 6:13,
10:29
Health hazards, 2:32, 2:36, 4:26,
6:5, 7:16, 7:19, 7:26, 7:28, 7:35,
7:36, 7:41, 8:35, 8:54, 8:63,
8:67, 8:78, A-5. See also Radio-
active poisons
Heavy water, 3:14, 5:6, 5:12,
5:14. As moderator, 2:10, 2:13,
2:28, 4:38, 4:47, 5:14, 6:8,
6:43, 6:45, 8:32, 8:55. Produc-
tion, 4:39, 9:36
Helium, see Alpha particle cooling
Hooker Electrochemical Co., 10:29
Houdaille-Hershey Corp., 10:26
Indiana, University of, 4:11, 6:38
Indium, 4:6
Iodine, 4:6
Ion accelerator, 1:40. See also
cyclotron
lonization chamber, 1:7, 4:6,
12:35, A-l
Iowa State College, 3:12, 6:16,
6:33, A-4
Isobars, 1:14
Isotopes, 1:14, 1:29. Separation
of, 2:12, 5:7, 5:15, 6:2, 9:lff.,
11:1
J. A. Jones Construction Co., Inc.,
10:24, 10:41
John and Mary Markel Founda-
tion, 11:12
Johns Hopkins University, 1:53,
3:12
Joint New Weapons Committee,
5:26
Kaiser Wilhelm Institute, 3:6
Kellex Corp., 9:32, 10:2, 10:24,
10:26, 10:41
Lattice, 2:11, 2:19, 2:31, 4:4, 4:6,
4:8, 4:13, 4:15, 4:16, 4:20, 4:22,
4:23, 4:27, 5:15, 6:21, 6:22,
6:28, 6:42, 6:43, 7:1, 7:3, 7:18,
7:19, 7:46, 8:4, 8:9, 8:21, 8:28,
8:36, 8:59, A-4. Poisoning of,
8:15
Lithium, 1:19, 1:38, 1:48, 2:10,
12:44
Los Alamos, N. M., 7:48, 12:4,
12:18, 13:1, A-6
Mallinckrodt Chemical Works,
6:12, 6:18, 6:19, 6:22
Manhattan District, 5:23, 5:29,
5:30, 5:32, 6:18, 7:4, 10:23,
11:11, 11:39, 12:2, 12:7, 13:1
Mass, conservation of, 1:2, A-l
Mass-Energy equivalence, 1 :4, 1 :6,
1:8, 1:38
Mass number, 1:13, 1:15, 1:35
Mass spectrograph, 1:35, 9:28ff.
Metal Hydrides Co., 4:41, 6:10,
6:15, A-4
Metallurgical Laboratory, 6:2,
6:33, 6:35, 7:1, 7:9, 7:10, 7:40,
7:46, 7:50, 8:2, 8:22, 8:34,
8:56, 11:2, 11:19, 12:2, 12:32.
See also Chicago, University of
Military Policy Committee, 5:25,
7:1
Minnesota, University of, 3:12,
6:38
Moderator, 2:8ff., 2:11, 2:13,
2:14, 2:19, 2:20, 2:28ff., 4:1,
262
Index
Moderator, 4:8, 8:8, 12:40. See
also Beryllium, Graphite, Heavy
water
Multiplication factor, 4:13, 4:15,
4:16, 6:10, 6:11, 6:22, 7:19,
8:12, 8:15, A-4
M. W. Kellogg Co., 10:2, 10:23,
10:32, 10:11. See also Kellex
Corp.
National Bureau of Standards,
3:8,3:12,4:36,4:41,4:44,6:11,
6:16, 6:20, 6:38, 9:31
National Carbon Co., Inc., 6:20,
10:23, A-4
NDRC, 2:37, 3:4, 3:9, 3:10.
Section S-l, 3:14, 4:14, 4:31,
5:2. See also Advisory Commit-
tee on Uranium
Naval Research Laboratory, 4:36,
11:38, 11:39, 11:47
Navy Department, 2:1, 3:4, 3:6,
3:11, 3:12, 4:36, 10:2, 11:39,
11:47
Neptunium, 1:58, 2:19, 6:34, 8:18
Neutron, l:18ff., l:23ff., 1:33,
1:49,2:3,2:6,2:19,2:32,12:16.
Absorption of, 1:47, 2:13, 1:57,
2:3, 2:8ff., 2:12, 2:19, 8:4, 8:6,
12:8, 4:8, 4:19, 6:20ff., A-5.
Delayed, 6:23, A-3, A-4. Detec-
tion, see Detection of nuclear
particles. Fast, 1:57, 2:1, 2:21,
2:14, 2:10, 2:12, 4:25, 6:37, 8:9.
Fission induced by, 1:52, 1:57,
2:21, 3:6, 4:6, 4:24, 4:25, 10:1.
Resonance, 2:12, 4:6, 4:8.
Sources, 1:40, 4:10, 4:14, 8:31,
8:32, 12:38, A-5. Thermal,
1:57, 2:1, 2:9, 2:21, 2:14, 3:6,
4:6, 4:25, 8:31, 8:39, 10:1,
12:40, A-l
Nitrogen, 1:17,1:51
Nuclear disintegrations, 1:15, 1:20,
1:38, 1:49. Artificial, 1:17
Nuclear Reaction, l:38ff. Nota-
tion for 1:43. See also Fission,
Nuclear disintegration, Nuclei
involved
Nucleus, 1:11, 1:12. Structure of,
l:23ff.
Oak Ridge, Tenn., see Clinton
Engineer Works
OSRD, 5:2, 5:9, 10:2, 10:23,
11:26, 12:2. Section S-l, 5:3,
5:17, 5:28, 7:1, 10:33, 11:4,
11:18. See also Advisory Com-
mittee on Uranium
Oxygen, 1:17, 1:51
Philadelphia Navy Yard, 11:39,
11:47
Pile, 2:20. See also Argonne Labo-
ratory, Clinton Engineer Works,
Hanford Engineer Works, Lat-
tice, West Stands pile
Planning Board, 5:5, 5:6, 6:8
"Pluto," 8:68
Plutonium, 1:58, 2:23, 2:36, 3:17,
4:24, 4:25, 5:7, 5:21, 6:1, 6:4,
6:34, 6:35, 8:lff., 12:10. Fission,
1:58, 2:14, 4:24, 4:48. Produc-
tion, 2:19, 5:14, 6:32, 6:41, 7:3,
7:8ff., 7:18ff., 8:lff., 9:44, 11:32,
2:18. Radioactivity, 1:58, 4:24
Poison gas, 4:27
Polonium, 1:19
Positron, 1:21, 1:51
Power, 1:48, 2:3, 2:16, 2:34, 3:5,
3:14, 4:48, 6:29, 7:26, 13:5
Princeton University, 3:12, 4:7,
4:9, 4:11, 4:12, 4:18, 4:22, 5:12,
9:40, 11:2, 11:44
Protoactinium, 1:57, 2:21
Index
263
Proton, 1:13, 1:17, 1:19, 1:23,
1:24, 1:33, 1:38, 1:40, 1:48,
1:51, 4:10, A-l
Purdue University, 6:38
Radioactivity, 1:6, 1:7, 1:15, 2:32.
Artificial, 1:26, 1:30, 4:6, 4:26,
A-l. Elements showing, 1:16.
Measurement of, see Cloud
chamber, counter, Detection of
nuclear particle, Electroscope,
lonization chamber. Poisonous
effects, 4:26, 4:48; see also
Health. Products of, see Alpha
particles, Beta particles, Gam-
ma rays, Nuclear disintegra-
tion, Radioactive series
Radium, 1:15, 1:16, 2:24, 2:32
Reference Committee (National
Research Council), 3:3
Reflector (tamper), 12:13, 12:24,
4:18,4:19
Relativity, 1:4
Relaxation time, A-4
Reproduction factor, see Multipli-
cation factor
Research Corporation, 1 1 :4
Resonance escape probability, 8:10
Reviewing Committee (National
Academy of Sciences) 3:15,
3:16, 3:18, 3:21, 3:23, 4:23,
4:24, 4:27, 4:48, 6:6, 6:39, 7:1
Rhodium, 4:6
Rice Institute, 6:38
Richland, Wash., see Hanford En-
gineer Works
Rockefeller Foundation, 11:12
Scintillations, A-l
Shielding, 7:28, 8:67
"Sneezy," 8:68
Speer Carbon Co., 6:20, A-4
Standard Oil Development Co.,
3:12, 5:12, 9:41, 9:42
Stanford University, 6:38
Stone and Webster Engineering
Corp., 10:34, 11:19, 11:45
Tamper, see Reflector
Tennessee Eastman Corp., 11:19,
11:34, 11:45
Thermal diffusion method of iso-
tope separation, 4:36, 9:21,
11:37
Thermal utilization factor, 8:11
Thorium, 1:15, 1:16, 1:57, 2:21,
2:24, 2:25
Tolman Committee, 13:5, 13:6
Top Policy Group, 3:22, 5:9
Transuranic elements, 4:25
Union Miniere, 3:7
Union Carbide and Carbon Corp.,
6:18
United States Graphite Co., 4:42
Units, A-l
Uranium, 1:7, 1:15, 2:23, 2:24,
2:26, 2:27, 6:4, 6:34, 11:36.
Fission, 1:3, 1:52, 1:57, 2:1, 2:3,
2:10, 2:12, 2:13, 2:14, 2:32, 3:6,
8:8ff., 2:35, 2:36, 4:49, 8:5,
10:1, 8:9, 3:16. Isotopes, 1:16,
2:18, 2:4, 4:34, 4:47, 5:21, 6:1,
10:1, 4:24. Isotope separation,
see Isotope separation. Neutron
absorption, 2:3, 2:6, 2:11, 2:19,
4:2, 4:6, 4:7, 4:24, 4:25, 8:5.
Uranium production, 4:41, 6:10,
6:12ff.
Uranium Bromide, 1 1 :4
Uranium Committee (NDRC),
3:9, 3:14, 4:36, 9:30, 11:26,
11:29. See also Advisory Com-
mittee on Uranium
264 Index
Uranium Hexafluoride, 4:33, 4:36, Washington University (St. Louis),
9:14, 10:3, 10:11 6:34
Westinghouse Electric and Manu-
Virginia, University of, 3:12, 5:12, facturing Co . } 6:10 , 6:13, 6:14,
9:42 9:42, 11:19, 11:45, A-4
War Department, 7:5, 3:6, 3:12, West Stands pile, 8:27, A-4
5:9, 11:18 Wisconsin, University of, 6:38
